allow MAVEN_ARGS_APPEND to work for S2I builds

(See also here: https://issues.jboss.org/browse/CLOUD-2206)

Using MAVEN_REPOS= as part of an S2I build never appear to have any effect for an EAP71 S2I build, for example building the EAP quickstarts from:

https://github.com/jboss-developer/jboss-eap-quickstarts/tree/7.1.0.GA

Requires that maven also enable the jboss-community-repository.

You can force this build to complete by adding MVN_ARGS_APPEND="-Pjboss-community-repository", but I think this is what MAVEN_REPOS=jboss-community-repository should be doing.

(https://github.com/jboss-openshift/cct_module/blob/master/os-eap-launch/added/launch/maven-repos.sh#L79)