Fix alternative tags processing.

One can now set on testsets (in profile attribute) and tests (in
docstring) alternative deployment tags in form 'tag_one | tag_two'.
Nose discovery plugin was modified in order to support new logic. New
utility function process_deployment_tags was added to nose_utils.py file.
This function now incapsulates all logic processing deployments tags in
means that it provides logical test whether particular testset and its
tests must be discovered for current cluster or not.

Change-Id: I10199a1f20d6edfbe25f1be24e4d5662101d1b99