use a LIST for the JMS connection-factory's connector attribute

The messaging's resources connection-factory and pooled-connection-factory use a MODEL type to represent their connectors.
This is for legacy version, as the first version of HornetQ that was integrated was required to specify a backup connector for each live connector. This was then discarded but the type of the attribute was not changed.

Currently, the connector attribute is represented as a DMR Object but only its keys are meaningful:

"connector" => {
"in-vm" => undefined,
"foo" => undefined
}

This complicates adding resources from the command line.
Now that we support resource transformations, the connector attribute should be represented as a list of STRING (and converted to its old representation for legacy version) so that the users can simply specify:

"connector" => ["in-vm", "foo"]

Since the CLI does not validate attribute type (it only check their names), the current code already accept a LIST of STRING such as above. However this is only documented in the attribute description field (its type remains an OBJECT)