gpperfmon drop partition sql statement was syntactically incorrect, so
partition_age gpperfmon feature was not working.

We were using the rows in partitionrangestart column from
pg_partition to drop specific partitions. The row value from
partitionrangestart is reported, as such, '2017-02-01 00:00:00'::timestamp(0) without time zone.
The query was reporting an error "Not a constant expression".

Use only the first part of partitionrangestart to make our ALTER DROP
query work.

- Added behave test to confirm that it is now working
Signed-off-by: NNadeem Ghani <nghani@pivotal.io>
Signed-off-by: NMarbin Tan <mtan@pivotal.io>
Signed-off-by: NC.J. Jameson <cjameson@pivotal.io>