Add solr::decommission class & apply it to vanadium

<ori-l> Nikerabbit: is it ok to remove role::solr::ttm from vanadium, btw?
<Nikerabbit> ori-l: it has been moved to a new server, but I already forgot the name

Also adds a bit of documentation for role::solr::ttm.

Change-Id: I6db14149b8f85c322ed41564d5a2ff315d2ed42d
Merge "Add solr::decommission class & apply it to vanadium" into production