replace all Job/JT relations with a single M2M credentials relation

Includes backwards compatibility for now-deprecated .credential,
.vault_credential, and .extra_credentials

This is a building block for multi-vault implementation and Alan's saved
launch configurations (both coming soon)

see: https://github.com/ansible/awx/issues/352
see: https://github.com/ansible/awx/issues/169

awx/main/migrations/_multi_cred.py

@@ -0,0 +1,12 @@
+def migrate_to_multi_cred(app, schema_editor):
+    Job = app.get_model('main', 'Job')
+    JobTemplate = app.get_model('main', 'JobTemplate')
+
+    for cls in (Job, JobTemplate):
+        for j in cls.objects.iterator():
+            if j.credential:
+                j.credentials.add(j.credential)
+            if j.vault_credential:
+                j.credentials.add(j.vault_credential)
+            for cred in j.extra_credentials.all():
+                j.credentials.add(cred)