purge: do not reboot by default

purge: do not reboot by default

Rebooting servers is really intrusive and perhaps this is not what the
operator wants. So we disable the reboot by default now. Note that the
reboot might not happen all the time.
It can be enabled by default by running the purge playbook with -e
reboot_osd_node=True

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1505011
Signed-off-by: Sébastien Han <seb@redhat.com>
(cherry picked from commit 2837d0a22e258cee583f14e402a99d89c9a16cd6)
Signed-off-by: Sébastien Han <seb@redhat.com>