refactor inventory Loaders for ansible-inventory

* remove support for loading from executable and static files
* instead use ansible-inventory with fallback to backport
* provide private file dir in task manager for cred injection
* durable management of tmp dirs for user scripts

* new 'scm' source choice for scm-type
* update SCM inventory docs to new reality

docs/scm_file_inventory.md

@@ -11,6 +11,12 @@ Fields that should be specified on creation of SCM inventory source:
  - `source_path` - relative path inside of the project indicating a
    directory or a file, if left blank, "" is still a relative path
    indicating the root directory of the project
+ - the `source` field should be set to "scm"
+
+Additionally:
+
+ - `source_vars` - if these are set on a "file" type inventory source
+   then they will be passed to the environment vars when running
 
 A user should not be able to update this inventory source via through
 the endpoint `/inventory_sources/N/update/`. Instead, they should update
@@ -40,18 +46,26 @@ update the project.
 
 > Any Inventory Ansible supports should be supported by this feature
 
-This statement is the overall goal and should hold true absolutely for
-Ansible version 2.4 and beyond due to the use of `ansible-inventory`.
-Versions of Ansible before that may not support all valid inventory syntax
-because the internal mechanism is different.
+This is accomplished by making use of the `ansible-inventory` command.
+the inventory import tower-manage command will check for the existnce
+of `ansible-inventory` and if it is not present, it will call a backported
+version of it. The backport is maintained as its own GPL3 licensed
+repository.
 
-Documentation should reflect the limitations of inventory file syntax
-support in old Ansible versions.
+https://github.com/ansible/ansible-inventory-backport
 
-# Acceptance Criteria Notes
+Because the internal mechanism is different, we need some coverage
+testing with Ansible versions pre-2.4 and after.
+
+# Acceptance Criteria Use Cases
 
 Some test scenarios to look at:
- - Obviously use a git repo with examples of host patterns, etc.
+ - Test projects that use scripts
+ - Test projects that have multiple inventory files in a directory,
+   group_vars, host_vars, etc.
+ - Test scripts in the project repo
+ - Test scripts that use environment variables provided by a credential
+   in Tower
  - Test multiple inventories that use the same project, pointing to different
    files / directories inside of the project
  - Feature works correctly even if project doesn't have any playbook files
@@ -61,3 +75,43 @@ Some test scenarios to look at:
  - If the project SCM update encounters errors, it should not run the
    inventory updates
 
+# Notes for Official Documentation
+
+The API guide should summarize what is in the use details.
+Once the UI implementation is done, the product docs should cover its
+standard use.
+
+## Update-on-launch
+
+This type of inventory source will not allow the `update_on_launch` field
+to be set to True. This is because of concerns related to the task
+manager job dependency tree.
+
+We should document the alternatives for a user to accomplish the same thing
+through in a different way.
+
+### Alternative 1: Use same project for playbook
+
+You can make a job template that uses a project as well as an inventory
+that updates from that same project. In this case, you can set the project
+to `update_on_launch`, in which case it will trigger an inventory update
+if needed.
+
+### Alternative 2: Use the project in a workflow
+
+If you must use a different project for the playbook than for the inventory
+source, then you can still place the project in a workflow and then have
+a job template run on success of the project update.
+
+This is guaranteed to have the inventory update "on time" (by this we mean
+that the inventory changes are complete before the job template is launched),
+because the project does not transition to the completed state
+until the inventory update is finished.
+
+Note that a failed inventory update does not mark the project as failed.
+
+## Lazy inventory updates
+
+It should also be noted that not every project update will trigger a
+corresponding inventory update. If the project revision has not changed
+and the inventory has not been edited, the inventory update will not fire.