Enforce no unused imports via Checkstyle

Removed with:

mvn checkstyle:checkstyle --quiet -Dcheckstyle.output.file=/dev/stdout -Dcheckstyle.output.format=plain | tac | while read i; do echo $i | sed -n 's/\([^:]*\):\([^:]*\):.*/sed -i \2d \1/p' | bash; done
JCLOUDS-393. Move to GCE API v1.
Responding to a @demobox comment