Allow customizing setup and create commands in integration tests

[johnstcn]

Looks like we both hit the same issue 😄 (I fixed it doing this: cd2cf5a)

I think I prefer your solution though 👍 (but not entirely sure why CODER_TASK_ID is being passed through as a uuid.Nil)

I think it's because we're creating a workspace and not a task in the integration test 🤦

			_, rc = execContainer(ctx, t, ctrID, fmt.Sprintf(`coder create %s -t %s %s --yes`, tt.name, tt.name, includeParameters))

Originally posted by @johnstcn in #456 (comment)

It would be useful to be able to customize the commands used to set up the template and create the resource in the integration tests. These are currently hard-coded.