tags should start with "mrjob:" not "__mrjob"

[coyotemarin]

AWS's own tags start with aws:. We should do the same with mrjob tag names, e.g. mrjob:version rather than __mrjob_version and mrjob:pool:name rather than __mrjob_pool_name.

Old tag names used by mrjob audit-usage, mrjob terminate-idle-clusters, etc. should still be added as well until mrjob 0.7.0.