mgr/dashboard: Use new ImageSpec class

mgr/dashboard: Use new ImageSpec class

This class should be used when dealing with RBD image specs.
It allows the creation of an ImageSpec given the names of the pool, image and
namespace. Alternatively you can also create one with an already existing
image spec string.

With it you keep the access to each individual component and can also convert it
to a well formated string.

Fixes: https://tracker.ceph.com/issues/42787
Signed-off-by: Tiago Melo <tmelo@suse.com>