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
Why repeating 1024 so many times? can't it be just

```instances_sz = ((disk_avail * 1024 ** 3) - instances_sz_bytes) / 1024 ** 3``` 
?