Remove service type restriction from keystone client help text

The current output of "keystone help service-create" implies that the
type is restricted to certain values.  This commit updates the help
text to make it more clear that the value isn't validated.

Closes bug 1248658
Closes bug 1211292

Change-Id: Ice4bf0da135d371ab5b6339f7baac7309e9c40ce
Merge "Remove service type restriction from keystone client help text"