[WFLY-1796] simplify write connector attribute

For backwards compatibility, the connector attribute (used in the
messaging subsystem's connection-factory & pooled-connection-factory)
must be a map where the key is a connector name and the value is not
taken into account (in previous HornetQ versions, the value was the
backup's server connector).

This is a source of confusion when creating resources with connector:
users expect to pass a list of connectors and this fails as they must
pass a map with undefined values.

This corrector will replace a list with the map expected to populate the
model.