Adding support for namespacing in credentials.properties.

Out-of-the-box configuration with this module supports the jetty webconsole, but webconsole fails to find ${activemq.username} and ${activemq.password} in credentials.properties. Default configuration specifies activemq.username in this file. Current activemq::user type uses the username specified to create entries like system.user=system, which don't allow this user to be added. Adding $namespace parameter to allow creating of entries of like activemq.username=system. $namespace defaults to $username, unless explicitly set, so should be backwards compatible.