WIP: Save operation

This is work in progress task for adding "save" operation into atmosphere. We agreed that "save" operation will only replace VMTs with new saved VM. It will not update information about AT `name`, `description`, `port_mapping_templates`, `endpoints`, `initial_configurations` and this information will be presented in the API doc.

Before it can be merged into master following actions need to be finished:

  - [x] VMT versioning
  - [x] migration adding version into VMT should fill this value with number 1 for all VMT managed by atmosphere
  - [x] Save action
  - [x] Remove VMT older than version
  - [x] 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
  - [ ] Save REST API documentation
Useless assignment to variable - `at`. Use `_` or `_at` as a variable name to indicate that it won't be used.