Add deployment-scanner for singleton deployments

To simplify the use of singleton deployments in standalone mode, it would be nice to have a mechanism analogous to the hasingleton-deploy directory in AS6 and earlier. To do this, we can enhance the existing deployment-scanner to accept an additional policy attribute. Something like:

<subsystem xmlns="urn:jboss:domain:deployment-scanner:1.1">
<deployment-scanner path="deployments" relative-to="jboss.server.base.dir" scan-interval="5000" policy="standard"/>
<deployment-scanner path="singleton-deployments" relative-to="jboss.server.base.dir" scan-interval="5000" policy="singleton"/>
</subsystem>