define native CredentialType inputs/injectors in code, not in the DB

This has a few benefits:

1.  It makes adding new fields to built-in CredentialTypes _much_
    simpler.  In the past, we've had to write a migration every time we
    want to modify an existing type (changing a label/help text,
    changing options like the recent become_method changes) or
    when adding a new field entirely

2.  It paves the way for third party credential plugins support, where
    importable libraries will define their own source code-based schema

awx/main/models/__init__.py

@@ -16,7 +16,7 @@ from awx.main.models.organization import (  # noqa
     Organization, Profile, Team, UserSessionMembership
 )
 from awx.main.models.credential import (  # noqa
-    Credential, CredentialType, V1Credential, build_safe_env
+    Credential, CredentialType, ManagedCredentialType, V1Credential, build_safe_env
 )
 from awx.main.models.projects import Project, ProjectUpdate  # noqa
 from awx.main.models.inventory import (  # noqa