glusterd-op-sm.c (and others) - improve glusterd_op_stage_set_volume()

Multiple changes to the function in the hope to make it somewhat faster.
1. Checking for key length against constant strings before calling
strcmp() to save some calls.
2. Verifying if a match was already made against the key to reduce
yet more checks.
3. Alignment of error message when they can fit on less lines - just
makes 'grep' on the code for error messages easier and it's more
readable.
4. Multiple functions where call _gd_get_vmep() one by one.
Instead, extracted it to be callable (it was static) and re-used
its result, instead of calling it again and again.
5. Removed some unneeded include statement.
6. Removed redundant null checks.

Hopefully, no functional changes.

Change-Id: Id281224e49adeca6757f96653b4cb13c7c9ba8c9
updates: bz#1193929
Signed-off-by: Yaniv Kaul <ykaul@redhat.com>