Rudder plugins

Maven is used at each "make" in plugins. It is dead slow and rather unpleasant to use.

Moreover, maven options are not used homogenuously in make scripts.

=> create a cache for M2_LOCAL_REPOS. Just putting it in a file (with an option to remove it with with "make very-clean" for ex).

=> DO NOT use resered env var MAVEN_OPTS

=> User everywhere MVN_CMD in place of mvn ...