core: Consolidated similar validations in add/update storage connection CDA

AddStorageServerConnection & UpdateStorageServerConnection command both
contained a lot of similar validations in the CDA part, all those validations
were extracted to the parent class for better readability and easier
maintenance

Change-Id: Id90cea934e0a9c10c5df435f564530b8658a64bb
Bug-Url: https://bugzilla.redhat.com/1129597
Signed-off-by: Tal Nisan <tnisan@redhat.com>