AuthMonitor: make code for updating caps reusable

AuthMonitor: make code for updating caps reusable

Move the code for "ceph auth caps" command to a separate function so
that the code can be reused to update caps.

Most of the code here is same as the code for creating new entity, so
let's modify this method to also create an entity. Also, create helper
methods to update and create an entity.

Signed-off-by: Rishabh Dave <ridave@redhat.com>