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.

Change-Id: I3abba1d9509bee649aa1675beee4153341b1e170