rolling_update: nicer way to set osd flags

rolling_update: nicer way to set osd flags

Prior to this patch, we were applying the osd flags like this:

"
General pre tasks
Set flags
Upgrade OSDs on a host
Unset flags <-- this triggers pending scrub to start
Set flags
Upgrade OSDs on a hosts
Unset flags <-- this triggers pending scrub to start
.
.
.
General post tasks
"

Now instead, we apply the flag once before starting the OSD update and
unset them once the last OSD is finished.

"
General pre tasks
Set flags and wait for any scrubs to finish
Upgrade OSDs on a host
Upgrade OSDs on a host
.
.
.
Unset flags
General post tasks
"

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1450754
Signed-off-by: Sébastien Han <seb@redhat.com>
Co-Authored-by: Guillaume Abrioux <gabrioux@redhat.com>