Create system tests for vcenter verification

Added:
Create system tests for vcenter support and partially enhance some
classes/functions
1. In os_actions.py - some functions for nova client
2. In fuel_web_client.py - support of vCenter deploying
3. In run_tests.py - import tests for vCenter
4. In settings.py - import vCenter settings from environment
5. In base_test_case.py - new deploy case with one slave

Implements: blueprint vcenter-hv-full-scale-support
Change-Id: I18991dbb4b63bf9f407acdd05422032014889b3d
Merge "Create system tests for vcenter verification"