Environment Interpolation

In influxdb & telegraf, you can substitute in environment variables with the `$VAR` syntax. This throws an error in kapacitor. My test case is changing this line in kapacitor.conf:

`bind-address = ":$PORT"`

Which then errors out on start (With the PORT env variable set)

Start with configuration loading for kapacitor.conf and reproduce the startup failure using bind-address = ":$PORT" with PORT set. Trace how the value is parsed, then verify that startup accepts the setting and uses the environment value for the bind address.

Written by the indexing model from the issue text.