Add updated_at field to app data model
Add updated_at field to app data model

Currently updated_at field is missing in the app show
response. This change adds field to the data model.

Change-Id: I1f8203d3c716991c64051fdd584a8d9c1c769463
Partial-Bug: #1583686

openstack/solum,stackforge/solum,stackforge/solum,openstack/solum