Allow setting the config path through an environment variable

[jsternberg]

The config path previously could only be specified through the command
line options. This made it very difficult to set a default config path
without using any option.

Now the environment variable can be set so the default configuration
path is set to a specific place, but can be overwritten using the
command line option.

The primary purpose of this is so the Docker container can have a
default configuration file, but not have to parse the command line
options to figure out if a different configuration file has been
specified while still allowing the user to only type influxd and have
the program start correctly.

This might also help #6392 as it would allow a default configuration
location to be included with the package by setting an environment
variable.

[jsternberg]

@jwilder @beckettsean

This change was primarily needed for the Docker image in the future, but I thought it might also be relevant to #6392.

[jwilder]

Might be nice to have a default config search path. e.g if no -config is specified, look for ~/.influxdb/influxdb.con, then '/etc/influxdb/influxdb.conf and fall back to default config if none exist.

The docker image wouldn't need to specifiy the -config flag if you mount the config into /etc/influxdb with the correct name then.

Env var as you have in the PR is still useful though. So 👍 on green.

[jwilder]

Needs changelog under features though.

[jsternberg]

@e-dard this is useful for the Docker images. We won't be able to include it in the 0.12 images, but it should make the entrypoint script more simple for 0.13.

[jsternberg]

@jwilder I'll add that in. What should be the order that's checked?

1. CLI option
2. Environment variable
3. Default search path

[e-dard]

LGTM 👍

[jwilder]

@jsternberg 3, 2, 1. More specific options overrides. The current config options work by using the config file, env var and finally CLI flags override those. Should work consistently with the existing model.

[jwilder]

👍