BUG-272: Fixed last checkstyle offenders & activate checkstyle enforcement

This patch adds xtend-gen to excluded directories and finally removes
the enforcer plugin override -- thus making md-sal components subject to
controller-wide checkstyle rules.

Change-Id: I3ccad778f90e450843122bc904a30a0cd4ca79f3
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>