Add MAVEN_MIRROR_URL parameter to templates

Would be great to be synced with EAP templates (as they have MAVEN_MIRROR_URL param https://github.com/jboss-fuse/application-templates/blob/application-templates-2.1.fuse-730043-redhat-00001/quickstarts/eap-camel-amq-template.json#L161), but Karaf and Spring Boot don't.

Note i've already prepared such change (and i'm currently testing it) in commit.

But i see two potential problems:

1. If user sets MAVEN_MIRROR_URL, it doesn't work by default, as there is set settings.xml provided by the quickstart itself (which we have) - which can be seen here - needs to be workarounded by specifying MAVEN_ARGS_APPEND to -gs /home/jboss/.m2/settings.xml - see comment
2. The mirror is not propagated to karaf's etc/org.ops4j.pax.url.mvn.cfg