add database indexing on counter_name for mongodb
limit normal get sample request to 100, and add get metername api
change the default sample list to 20 entries
modify mongodb driver to: 1) fix bugs related to openflow counter max out; 2) fix bugs with image metadata has a field with dot; 3) added indexing for improve query performance
Merge branch 'devel' of https://github.com/jyericlin/ceilometer into devel