cli: add messages to assertTrue

1) Unify assertion of 'usage: service-name' from
   {cinder, glance, keystone, neutron} help tests
   into assertFirstLineStartsWith method, which provides
   more usefull failure message.
2) Add failure message to assertTrue in keystone-catalog
   test.

(cherry picked from commit 4c18fa1bbf85e3e8d0352b45a6f235afaeea4081)

Conflicts:
	cli/__init__.py
	tempest/cli/simple_read_only/test_cinder.py
	tempest/cli/simple_read_only/test_glance.py
	tempest/cli/simple_read_only/test_neutron.py

Change-Id: I3abba1d9509bee649aa1675beee4153341b1e170
Merge "cli: add messages to assertTrue" into stable/grizzly