mgr/dashboard: Fix 500 error while exiting out of maintenance

mgr/dashboard: Fix 500 error while exiting out of maintenance

When you add a host in maintenance mode and then exit the maintenance
mode, a 500 server error will popup which will interrupt the whole
exit maintenance process and leave the host in an unknown/offline state.
It happened when I was setting the status of the host through the
HostSpec(). With this change, I am using the enter_maintenance api of
the orch to enable the maintenance.

Fixes: https://tracker.ceph.com/issues/51218
Signed-off-by: Nizamudeen A <nia@redhat.com>