core: add validation to AddStorageServerConnection

Prevent addition of a new connection if another one with same
connection details already exists.
Added more unitests for the AddStorageServerConnection command.
Also added VAR__TYPE__STORAGE__CONNECTION to VdcBllMessages, so the connection related
commands will return a proper message with correct entity type (instead
of returning var type storage domain). AppErrors files already contain
this var type, so no need to add it there.
Also, fixed the failure handling of storage domain creation in webadmin UI
-  if connection command failed and no connection was
created, there is no need to send command to backend to delete the
connection.

Change-Id: If9bcef8a413589654d36db8d878c844550ae6066
Signed-off-by: Alissa Bonas <abonas@redhat.com>