<Admin::Stats> fix minor bug - NONE district should be HashWithReaders
<Admin::Stats> exclude inactive nodes from eff avail gears

district_summary[:effective_available_gears] should be the number of
gears that are really available for creating new active gears in a
district. So, it now excludes nodes marked inactive in the district.
NONE districts are excluded from this logic since they don't really
have active/inactive nodes.
<Admin::Stats> use only strings for hash keys

This also enables using JSON for serialization - previously it didn't
record the difference between symbols and strings; now there are only
strings, so we can read JSON in usefully with oo-stats.
Merge pull request #3413 from sosiouxme/admin-stats-fixes

Merged by openshift-bot