Add forge token env

[scotje]

@logicminds it wouldn't let me push to your fork, but I took your commit and changed a couple things:

• renamed the env var to be more consistent with existing PDK env vars
• removed the default value from the CLI help and instead just fall-back to env if CLI value is empty
• use the PDK::Util env helpers
• added unit tests

Supersedes #912

[coveralls]

Coverage increased (+0.005%) to 92.654% when pulling a42cafa on scotje:add_forge_token_env into 226a101 on puppetlabs:master.

[logicminds]

The cli argument is required and I suspect the cli will fail even if the env variable is set because no default is supplied. Was this covered in your test?

[scotje]

@logicminds argument: required on a CLI option just means that, if you specify that option, you must include a value. So, you can't just say pdk release publish --forge-token, you would have to say pdk release publish --forge-token=foo. But it does not mean that the option itself is required.

$ pdk release publish
pdk (ERROR): You must supply a Forge API token either via `--forge-token` option or PDK_FORGE_TOKEN environment variable.

$ pdk release publish --forge-token
publish: option requires an argument -- forge-token

$ PDK_FORGE_TOKEN=abc123 pdk release publish
pdk (INFO): Releasing jesse-testmod - from version 0.1.0
pdk (INFO): Uploading tarball to puppet forge...