Support SSL /TLS support for graphite in influxdb

__Proposal:__
As we need to send metrics from graphite-based clients securely we would need the influxdb (1.8) to support SSL / TLS for graphite endpoint as well. Currently its seen for influx-line and openTSDB endpoints.

__Current behavior:__
No documentation indicating any configuration for the same.

__Desired behavior:__
If support is present explicit documentation or if possible adding the feature.

__Alternatives considered:__
Telegraf, however, given the load. 30K ~ 50K Wites per second, Not sure if telegraf can be the way.

__Use case:__
Being able to have secure graphite client connections to influx in v1.8

Start by checking InfluxDB 1.8's existing SSL/TLS configuration for the line-protocol and OpenTSDB endpoints, then trace how the Graphite endpoint is configured. Done means either documenting the existing Graphite TLS configuration explicitly or implementing secure Graphite client connections if support is absent.

Written by the indexing model from the issue text.