BUGFIX: PCI-3781 nova-compute service is running vgs multiple times

- refactoring part of code to get all needed values in one call instead of
  consequenti multiple call of the same expensive external command
- optimalizing calls of new methods and functions little bit
If we had something more recent than Folsom, you would not even have this discussion ;)
https://github.com/openstack/nova/commit/b823db737855149ba847e5b19df9232f109f6001