Update application_roles table for provisioning.

Adds cloud_id and claimed_until columns to the application_roles table
so that rows can be locked when they're being processed and updated with
relevant Azure information when processing is finished. Since the
claimed_until column is now common to a few different models, I've also
moved it to a mixin.

atst/models/mixins/claimable.py

@@ -0,0 +1,5 @@
+from sqlalchemy import Column, TIMESTAMP
+
+
+class ClaimableMixin(object):
+    claimed_until = Column(TIMESTAMP(timezone=True))