mon: OSDMonitor: allow (un)setting 'hashpspool' flag via 'osd pool set'

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 1c2886964a0c005545abab0cf8feae7e06ac02a8)

Conflicts:

	src/mon/MonCommands.h
	src/mon/OSDMonitor.cc

mon: ceph hashpspool false clears the flag
instead of toggling it.
Signed-off-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 589e2fa485b94244c79079f249428d4d545fca18

Replace some of the infrastructure required by this command that
was not present in Dumpling with single-use code.
Signed-off-by: Greg Farnum <greg@inktank.com>