engine: Introduce getConnectableStorageConnectionsByStorageType API

This API allows to retrieve all connectable storage connections by
storage type. If storage type is not specified then this API returns
the same result as getAllConnectableStorageSeverConnection -  all
connectable storage connections.

Change-Id: I9cea901ff2707bb4caa52e8f8eff06d533d5ddb6
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>