[releng] Enforce Maven version to 3.6.0

See Bug 551674.

This commit also moves the enforcer plug-in into the correct place
to be activated

The enforcer plug-in needs to be done to allow the check maven versions
to not report erroneous/extra information too (see next commit)

Change-Id: I2da89b0a9ec0fc3dc27f0812fef53104dcaa7fd6
1 file changed