Allow JTs to specify and prompt for SCM branch

Copy project folder each job run
  change cwd to private_data_dir, from proj
  do not add cwd to show_paths if it is
  a subdirectory of private_data_dir, which
  is already shown

Pass the job private_data_dir to the local
  project sync, and also add that directory
  to the project sync show paths

Add GitPython dep and use for job sync logic
  use this to manage shallow clone from desired
  commit, and to map branch to commit,
  and to assess necessity of project sync

Start on some validation change, but not all
  allow arbitrary playbooks with custom branch

awx/main/models/projects.py

@@ -261,9 +261,14 @@ class Project(UnifiedJobTemplate, ProjectOptions, ResourceMixin, CustomVirtualEn
     scm_update_cache_timeout = models.PositiveIntegerField(
         default=0,
         blank=True,
-        help_text=_('The number of seconds after the last project update ran that a new'
+        help_text=_('The number of seconds after the last project update ran that a new '
                     'project update will be launched as a job dependency.'),
     )
+    allow_override = models.BooleanField(
+        default=False,
+        help_text=_('Allow changing the SCM branch or revision in a job template '
+                    'that uses this project.'),
+    )
 
     scm_revision = models.CharField(
         max_length=1024,