Add snapshot restore HTTP API

This adds the external HTTP API and the RPC methods to invoke stack
restore.

blueprint stack-snapshot
Co-Authored-By: ala.rezmerita@cloudwatt.com

Change-Id: Id1ec0c11c9f12c23b3eec12a853db6ee4ff90277
Don't stop create immediately on error for stack resources

Don't stop create immediately on error for stack resources,
such as autoscalingGroup.

Kicked from Gate, heat can't pass pep8.

Change-Id: I0a41954f89e2000785c2105b8366e9e49858b65f
Closes-bug: #1383645
Merge "Add snapshot restore HTTP API"
Merge "Don't stop create immediately on error for stack resources"