Define MAVEN_LOCAL for unpublished Maven JARs

Standardize the definition of MAVEN_LOCAL as ~/.m2/repository.
This makes it slightly easier to point to a custom build of a
dependency such as gwtorm or PrologCafe by installing the JAR to
the local Maven repository and updating the maven_jar() rule to
use repository = MAVEN_LOCAL instead of GERRIT.

Change-Id: I2e54d1039608c0195a992dbc12fe74bb513466c6