Job scheduling: Add support for environment variables

[josegonzalez]

The easy way to do this is just to add it to the env stanza. While this isn't the most secure thing to do, for many installations, this is more than enough for many users that just want HA applications.

Note that built docker images for herokuish/buildpack deploys already vendor environment variables (which should obviously be fixed, but likely won't be until Buildpack V3 becomes stable enough to replace herokuish), so we're not actually making the system much less secure by hardcoding the environment variables in the job.

[josegonzalez]

Note that we should have the ability to turn on/off the environment variable injection. This will allow future releases of this plugin to inject custom binaries into the image to automatically pull environment variables from systems like Vault. An example implementation might be something like vaultenv.