core: Consolidated common test of add/update storage connection

AddStorageServerConnection and UpdateStorageServerConnection command tests
contained simiar tests, those common tests were extracted to a parent test
class for better readability and easier maintenance

Change-Id: Iad571ca0c5788c375200847e8872706a8dfd7f3e
Bug-Url: https://bugzilla.redhat.com/1129597
Signed-off-by: Tal Nisan <tnisan@redhat.com>