Add influxdb subscription path to allow for Kapacitor to run behind a reverse proxy

In the kapacitor. Conf file, the configuration urls for InfluxDB do not support path parameters, which may be unavailable if InfluxDB is reversely represented

eg:
```
[[influxdb]]
enabled = true
default = true
name = "current"
urls = ["http://127.0.0.1:8080/influxdb"]
```

Start by tracing how the kapacitor.conf InfluxDB urls setting is parsed and used for requests. Add coverage for an InfluxDB URL containing the /influxdb path, and consider the issue done when that configuration works for Kapacitor behind a reverse proxy without breaking existing URLs.

Written by the indexing model from the issue text.