userportal,webadmin: extract the ClientStorage interface

This patch makes the ClientStorage to be an interface (e.g. simply mockable)
and bound to a ClientStorageImpl class in the BaseSystemModule.

Change-Id: If5eaa942181c9df77278e736ddd6ac6b08e83fc7
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>