WIP: Save operation

This is work in progress task for adding "save" operation into atmosphere.

Before it can be merged into master following actions need to be finished:

  - [x] VMT versioning
  - [ ] Save action (this action should be asynchronous with precondition checked)
  - [ ] Remove VMT older than version
  - [ ] Monitoring task sets correct version when VMT is a copy of another VMT (such situation occurs during VMT migration into different compute site)
  - [ ] REST interface invoking Save action
Useless assignment to variable - `vmt`.