BUGFIX: PCI-3781 nova-compute service is running vgs multiple times for no reason

- 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
TEST: PCI-3781 update HostStateTestCase for new conditions
BUGFIX: PCI-3781 Missing pool_path variable

- adding back accidentally removed line
Merge pull request #39 from zdenekpizl/zpi-pci-3781

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