automatically encrypt/decrypt main_oauth2application.client_secret

see: https://github.com/ansible/awx/issues/1416

awx/main/models/oauth.py

@@ -9,6 +9,9 @@ from django.utils.translation import ugettext_lazy as _
 
 # Django OAuth Toolkit
 from oauth2_provider.models import AbstractApplication, AbstractAccessToken
+from oauth2_provider.generators import generate_client_secret
+
+from awx.main.fields import OAuth2ClientSecretField
 
 
 DATA_URI_RE = re.compile(r'.*')  # FIXME
@@ -39,6 +42,10 @@ class OAuth2Application(AbstractApplication):
         null=True,
     )
 
+    client_secret = OAuth2ClientSecretField(
+        max_length=1024, blank=True, default=generate_client_secret, db_index=True
+    )
+
 
 class OAuth2AccessToken(AbstractAccessToken):