Bump maven-checkstyle-plugin to 3.0.0

Bump maven-checkstyle-plugin to 3.0.0

https://mail-archives.apache.org/mod_mbox/maven-announce/201801.mbox/%3Cop.zchs68akkdkhrr%40desktop-2khsk44.mshome.net%3E

Also removes checkstyle.version property, as noted in its TODO, and
switches from <sourceDirectory/> to <sourceDirectories/>, as
documented on the plugin site on
https://maven.apache.org/plugins/maven-checkstyle-plugin/

Change-Id: I3bffa47a300f314aaefe5181ca92fd0a6c925755
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Stephen Kitt <skitt@redhat.com>