mon: allow overriding the initial mon_host

mon: allow overriding the initial mon_host

This overrides what the CephContext believes to be the current quorum of
monitors (retrieved from other instances of the MonClient), introduced
by [1]. Tests need to be able to target a specific monitor for
exercising forwarding and other things.

[1] 731e2db9fb4611f767446a3c8e778a097ce70d35
Fixes: https://tracker.ceph.com/issues/47180
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
Signed-off-by: Shyamsundar Ranganathan <srangana@redhat.com>
(cherry picked from commit ed3782e60afa0da2f523fc6df7b593fe7a57646a)

Conflicts:
	PendingReleaseNotes
Retained nautilus release notes with the required addition as above