add an endpoint for delete api (https://rbcommons.com/s/pepperdata/r/5336)
add an api endpoint for deleting metrics. this will be used for enforcing
metrics retention policy. for a given metrics query, delete the rows that match
the query.  this has certain ramifications. e.g. if you specify a time range
falling within an hour bucket, all data for that row (hour's worth) will be
deleted. start time and end time are required parameters. even if you specify a
start time aligned on hour boundary, it's possible that you will delete the
previous hour's data. this is because opentsdb constructs the start of the hbase
scan to account for interpolation window and downsampling interval. the default
interpolation window is 60 minutes, so it shifts the start back an hour. it's
best to specify a small interpolation window in the query and also specify a
time in the middle of the hour bucket to delete that hour's data.   additionally
fixed a flakey unit test (fsck_nodata) which was intermittently failing due to a
divide by zero error.  link to investigation of implementations for metrics data
expiration: https://docs.google.com/a/pepperdata.com/document/d
/1im64avxCI5IOhBphRdjgOgAnYo1ZWev5jpDY-9n6x2M/