Checkstyle enforce space-instead-tab control for *.xtend

Checkstyle enforce space-instead-tab control for *.xtend

https://git.opendaylight.org/gerrit/#/c/47975/ must be merged before
this one, otherwise the build will break (because that affects a project
which is already enforcing CS compliance).

I've verified, using a simplified version of the
https://wiki.opendaylight.org/view/EvolvingCheckstyle procedure, that
this tightening of CS rules has no impacts anywhere else (other than
c/47975).

Change-Id: I772391310907b5890b5e64e5039f689a2f91e520
Signed-off-by: Michael Vorburger <vorburger@redhat.com>