elasticsearch: better shard check output

previously icinga (or something in the middle) would strip single quotes,
mangling the output, e.g.

production-logstash-eqiad: {ustatus: uyellow, unumber_of_nodes: 3,
uunassigned_shards: 1, utimed_out: False, uactive_primary_shards: 36,
ucluster_name: uproduction-logstash-eqiad, urelocating_shards: 0, uactive_shards: 102,
uinitializing_shards: 0, unumber_of_data_nodes: 3}

Change-Id: Ibed17dde6e2e01e5613c5617654297c5ae55f684