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
ok, there are several occurrences of ** 3 above in the code. So apparently there is still plenty of room to make this script blazingly fast ;)