Add support for TFC Remote agents

[koikonom]

Allows the operator to create workspaces that use remote agents instead
of running terraform on TFC hosted agents.

See https://www.terraform.io/docs/cloud/workspaces/agent.html for more
information on agents.

In order to use the feature we need to add the following YAML snippet in
our workspace spec:

```
spec:
  #  ... other code ...
  agentPoolID: apool-XXXXXXXXXXXXXXXX
```

[koikonom]

How to test:

• Follow the instructions in this page to create an agent pool, get the token required: https://www.terraform.io/docs/cloud/agents/index.html#managing-agent-pools, and start the agent in a container. Once the agent starts you will see something like this:

2020-11-23T18:36:50.502Z [INFO]  core: Agent registered successfully with Terraform Cloud: id=agent-XXXXXXXX pool-id=apool-XXXXXXXXX

• Add the following line to your workspace spec:

spec:
  #  ... other code ...
  agentPoolID: apool-XXXXXXXXXXXXXXXX

The pool ID needs to match the pool ID you saw in the agent's startup messages.

• Apply the workspace spec and verify that the plan and apply steps run on the cloud agent by looking at its log output.

[dak1n1]

Thanks for this feature! It all works as expected.