Add in more model changes around execution environments

- a new unique name field to EE
- a new configure-Tower-in-Tower setting DEFAULT_EXECUTION_ENVIRONMENT
- an Org-level execution_environment_admin_role
- a default_environment field on Project
- a new Container Registry credential type
- order EEs by reverse of the created timestamp
- a method to resolve which EE to use on jobs

awx/conf/fields.py

@@ -14,6 +14,7 @@ from rest_framework.fields import (  # noqa
     BooleanField, CharField, ChoiceField, DictField, DateTimeField, EmailField,
     IntegerField, ListField, NullBooleanField
 )
+from rest_framework.serializers import PrimaryKeyRelatedField
 
 logger = logging.getLogger('awx.conf.fields')