mgr/k8sevents: Initial ceph -> k8s events integration

mgr/k8sevents: Initial ceph -> k8s events integration

Adds a mgr module to provide integration between
Ceph and the kubernetes events API within the rook-ceph
namespace. It provides several commands to view event
status including;

ceph k8sevents list .... show all k8s related events
ceph k8sevents ceph .... show events generated from this
                         module
ceph k8sevents status .. show status of the tracker threads
                         and counts of tracked events

Events sent to kubernetes are sourced from the
a) clog: ceph healtchecks and admin commands
b) explicit checks: hosts, pools and OSD states are checked
   every 'n' seconds

Signed-off-by: Paul Cuzner <pcuzner@redhat.com>