Code changes to avoid return duplicates from metric_name search

[ChandraAddala]

The metric_name/search API, sometimes returns duplicate data. This happens esp when the data is present in multiple indexes. Since we query from an alias in prod which points to multiple indexes this returns duplicate data as shown below. This is causing issues with grafana + blueflood integration. This PR addresses this issue by eliminating duplicates.

curl -XGET 'http://localhost:2500/v2.0/<tenantid>/metric_name/search?query=ord.instance_metrics.ord.a0001.xxxx.vif_0_rx'| python -m json.tool

[ 
    { "ord.instance_metrics.ord.a0001.xxxx.vif_0_rx": true },
    { "ord.instance_metrics.ord.a0001.xxxx.vif_0_rx": true }
]

[coveralls]

Coverage increased (+0.01%) to 74.938% when pulling 045154f on ChandraAddala:remove-duplicates-metric-name-search into 17b8b98 on rackerlabs:master.

[shintasmith]

lgtm