core: Added alert on number of LVs config value

Added a config value for the max number of LVs in a storage domain from
which every image creation will cause an alert to the user

Change-Id: Ie5ed86e41cb20c819cf8466c386027d8e0738590
Related-To: https://bugzilla.redhat.com/1015186
Signed-off-by: Tal Nisan <tnisan@redhat.com>