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/tests/functional/test_rbac_project.py

@@ -8,7 +8,7 @@ from awx.main.migrations import _old_access as old_access
 
 
 @pytest.mark.django_db
-def test_project_migration():
+def test_project_migration(credentialtype_ssh):
     '''
 
         o1  o2  o3       with   o1 -- i1    o2 -- i2
@@ -59,7 +59,7 @@ def test_project_migration():
     o2 = Organization.objects.create(name='o2')
     o3 = Organization.objects.create(name='o3')
 
-    c1 = Credential.objects.create(name='c1')
+    c1 = Credential.objects.create(name='c1', credential_type=credentialtype_ssh)
 
     project_name = unicode("\xc3\xb4", "utf-8")
     p1 = Project.objects.create(name=project_name, credential=c1)