host command replaced with ping one
1. compute_nodes param was added to config.py
2. compute_nodes param parsing was added to
   NailgunConfig in config.py
3. host 8.8.8.8 resolving was added after the
   successfull ping to DNS availability test
Minor fixes after test execution on OSLO lab:
1. Check that commant Exit status 1
	is not related to real fail added
	to the DNS test.
2. SSH client was refactored to return command
	output anywhere. Previously only error
	code was returned in case of error
OSTF-210: Review comments applied.
1. -w 1 param was added to ping command
	to reduce the time of waiting
2. Error code check was added to ping command
	verification instead of parsing the answer.
3. Comment about self.host cannot be applied because
	this code fragment looks like very obsolete
	(changed at bcc01fb87ab9b182ffac362a3e4b0357ec75700d
	commit, JUL-26)
reverting changes in config.py were made by mistake
Refactoring after test execution on VB vm

1. Obsolete params were removed from infrastructure test and config.py
   (looks like were added during rebase)
2. Infrastrucuture test class dicstring was updated to the
   current tests state
Old param fix in nmanager.py (rebase eror)
Merge pull request #55 from Mirantis/OSTF-210_dns_test_refactoring

Ostf 210 dns test refactoring