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/settings/defaults.py

@@ -281,6 +281,7 @@ REST_FRAMEWORK = {
     'VIEW_NAME_FUNCTION': 'awx.api.generics.get_view_name',
     'VIEW_DESCRIPTION_FUNCTION': 'awx.api.generics.get_view_description',
     'NON_FIELD_ERRORS_KEY': '__all__',
+    'DEFAULT_VERSION': 'v2'
 }
 
 AUTHENTICATION_BACKENDS = (