unit test for invokerAgent

[dgrove-oss]

The invokerAgent added in #155 should have some unit tests, especially of the log route.

Create some tests, possibly restructuring agent to make it easier to mock out the connections to docker/file system, and figure out how to set up go test to actually be able to run them. This may require changing the current build strategy, which is to do a multi-stage Docker build to avoid assuming that there is a Go toolchain installed on the developer's workstation. Or we could do a multi-stack docker build that includes testing as well...

[dgrove-oss]

With the removal of the log route from the invokerAgent, there isn't anything left that can be tested in a unit test. Only function left in the agent is the forwarding of suspend/resume to docker and that code is pretty trivial.