Implement function to manage/unmanage snapshots

Implement function to manage/unmanage snapshots

1. Add snapshots manage action in contrib api and unmanage
action in API extension.
2. Implement manage_existing_snapshot in manager and flow
to import existing snapshots.
3. Add manage/unmanage support in the LVM driver

Implements: blueprint support-import-export-snapshots
DocImpact
APIImpact
Similar to volume manage/unmanage.
Change-Id: Ib6cf8392b0bc99f803316991f31a75788677e9cf