Resolve environment without values to values on host

For parity with the Docker CLI, allow environment variables without
values to be automatically resolved to their values on the host.

Signed-off-by: Ryan Brainard <brainard@heroku.com>

Conflicts:
	tests/integration/service_test.py
Add documention for key-only environment

Signed-off-by: Ryan Brainard <brainard@heroku.com>
Merge pull request #297 from ryanbrainard/resolve-env

Resolve environment without values to values on host