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/api/versioning.py

@@ -1,10 +1,22 @@
 # Copyright (c) 2017 Ansible by Red Hat
 # All Rights Reserved.
 
+from django.conf import settings
+
 from rest_framework.reverse import reverse as drf_reverse
 from rest_framework.versioning import URLPathVersioning as BaseVersioning
 
 
+def get_request_version(request):
+    """
+    The API version of a request as an integer i.e., 1 or 2
+    """
+    version = settings.REST_FRAMEWORK['DEFAULT_VERSION']
+    if request and hasattr(request, 'version'):
+        version = request.version
+    return int(version.lstrip('v'))
+
+
 def reverse(viewname, args=None, kwargs=None, request=None, format=None, **extra):
     if request is None or getattr(request, 'version', None) is None:
         # We need the "current request" to determine the correct version to
@@ -13,7 +25,7 @@ def reverse(viewname, args=None, kwargs=None, request=None, format=None, **extra
         if kwargs is None:
             kwargs = {}
         if 'version' not in kwargs:
-            kwargs['version'] = 'v2'
+            kwargs['version'] = settings.REST_FRAMEWORK['DEFAULT_VERSION']
     return drf_reverse(viewname, args, kwargs, request, format, **extra)