Introduce a new CredentialTemplate model

Credentials now have a required CredentialType, which defines inputs
(i.e., username, password) and injectors (i.e., assign the username to
SOME_ENV_VARIABLE at job runtime)

This commit only implements the model changes necessary to support the
new inputs model, and includes code for the credential serializer that
allows backwards-compatible support for /api/v1/credentials/; tasks.py
still needs to be updated to actually respect CredentialType injectors.

This change *will* break the UI for credentials (because it needs to be
updated to use the new v2 endpoint).

see: #5877
see: #5876
see: #5805

awx/main/management/commands/create_preload_data.py

@@ -3,7 +3,7 @@
 
 from django.core.management.base import BaseCommand
 from crum import impersonate
-from awx.main.models import User, Organization, Project, Inventory, Credential, Host, JobTemplate
+from awx.main.models import User, Organization, Project, Inventory, CredentialType, Credential, Host, JobTemplate
 
 
 class Command(BaseCommand):
@@ -30,8 +30,12 @@ class Command(BaseCommand):
                         scm_update_cache_timeout=0,
                         organization=o)
             p.save(skip_update=True)
-            c = Credential.objects.create(name='Demo Credential',
-                                          username=superuser.username,
+            ssh_type = CredentialType.from_v1_kind('ssh')
+            c = Credential.objects.create(credential_type=ssh_type,
+                                          name='Demo Credential',
+                                          inputs={
+                                              'username': superuser.username
+                                          },
                                           created_by=superuser)
             c.admin_role.members.add(superuser)
             i = Inventory.objects.create(name='Demo Inventory',