no more custom monitor retention policy

__Steps to reproduce:__
According to following commits:

5 sept. 2015 Monitor retention policy is configurable https://github.com/influxdata/influxdb/commit/214cfea53c05fcc7105ca60d222572528153713c
Retention policy name is configured using store-retention-policy = "monitor"

11 sept. 2015 By default write stats to default retention policy https://github.com/influxdata/influxdb/commit/3cb29d92411251f58155d49d539425d43a22ff12
Retention policy is now using default retention policy

15 sept. 2015 Create and configure _internal retention policy https://github.com/influxdata/influxdb/pull/4108
A "monitor" retention policy is used as retention policy. Retention policy is no more customizable ar creation. Metrics are written into default retention policy.

17 feb. 2016 Set the retention policy on the store https://github.com/influxdata/influxdb/commit/08ca1487249719042bfe461100f4e41f127b7d5b
Metrics are written into "monitor" retention policy.

__Expected behavior:__
monitor retention policy expect to be customizable.

__Actual behavior:__
monitor retention policy is hard written.

__Environment info:__
* System info: Debian
* InfluxDB version: 1.7

Review the cited retention-policy commits and the store-retention-policy setting first. Trace where the monitor policy is hard-coded in InfluxDB 1.7, then verify that the setting controls it; the issue is done when monitor retention is configurable rather than hard-written.

Written by the indexing model from the issue text.