Passes environment variables to docker subprocesses

[jlashner]

Small change to the host-manager agent so that it passes the environment through to subprocess calls.

Motivation and Context

This is required for calling docker services that use environment variables.

How Has This Been Tested?

Tested by running the host_manager at Penn

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.