Add SITE_HOST envvar to ocs-agent-cli

[mhasself]

Description

See above.

Motivation and Context

Primary motivation is to make it easier to configure a single HostManager to control all agents on a physical host, including ones managed through docker and a docker-compose.yaml file.

In our new docker service definition scheme, environment variables are used to set basic agent configuration. This adds SITE_HOST to INSTANCE_ID, SITE_HUB, SITE_HTTP. The SITE_* ones can be defined in an anchor, and used throughout the docker-compose file; e.g.:

x-basic-env:
    &basic-env
    SITE_HOST: host1
    LOGLEVEL: debug

services:
  ocs-faker1:
    image: ocs:latest
    hostname: host1-docker
    environment:
      <<: *basic-env
      INSTANCE_ID: faker1
    network_mode: host
    volumes:
      - ./:/config:ro

Use of anchors in this way does require treating environment as a dict rather than a list.

How Has This Been Tested?

Directly by manipulating the configuration of the docker-compose.yaml. The argument remains optional, and has the correct default behaviors.

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.

[BrianJKoopman]

Looks good! Tested it out locally on my system.