mon/FSCommands: use alias for higher brevity

mon/FSCommands: use alias for higher brevity

Create an alias so "APP_NAME_CEPHFS" can be written instead of
"pg_pool_t::APPLICATION_NAME_CEPHFS".

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 4a12f6eb6c16722f1d68e0949390cd7d30f44bc6)

Conflicts:
	src/mon/FSCommands.cc
	* Conflict occured because variable in Reef is "fs" and in Main
	  branch is "fsp".