webadmin: allow to extend KeyValueModel

Created a base class for KeyValueModel (BaseKeyModel), that
extracts the logic of key handling.
Each derivative class will handle only values, and the way
it serialize and deserialize its data.
The motivation is to allow a new key model
for vm selection (depending patch).

Change-Id: I92d49a9e3a6099e0e082939bee61ccf484545263
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>