Unify the “--backend-parameters” option

This is not straight-forward. First, gnt-cluster had
“--backend-parameters” wheres the rest of the script had just
“--backend”. I settled on the full form, since one can always abbreviate
options, but not vice-versa.

Second, the help in some cases was different, e.g. start instance
had “Temporary backend parameters” and modify had “Change backend
parameters”. I think this kind of distinctions are better served in the
man page. Alternatively, if we could copy options easily, one could
re-instantiate them with a changed help line.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>