Support connecting to Docker via HTTPS

-RN
We will attempt an HTTPS connection if a path to a client.key/client.pem
pair is specified via the --docker-cert-path argument in the agent or via
the DOCKER_CERT_PATH environment variable.
Upgrade docker-client dependency
Fix tests that were failing against HTTPS Docker

These tests were failing against HTTPS Docker instances because they create
their own DockerClient without checking for HTTPS or certs.

Instead, have them delegate creation of the DockerClient to SystemTestBase,
which is aware of certs.
Update user_manual.md

Tell users how to install Helios CLI via apt or brew.
Log full error when task runner throws exception

This seems to be happening pretty often on our Jenkins build machines, so
it would be nice to have the full stacktrace and details to debug.
Allow specifying job prefix for temporary jobs

-RN
When building a TemporaryJobs, you can now specify a job prefix for all of
the jobs the TemporaryJobs creates.
Use consistent jobPrefix for helios-testing tests

When creating temporary jobs in the tests for helios-testing, ensure that
all the jobs have a consistent job prefix (i.e., `testTag`). This in turn
ensures that the jobs are cleaned up by SystemTestBase's teardown.

-RN
Ensure that the jobs are cleaned up by SystemTestBase's teardown.
Rename helios-tools Debian package
Upgrade docker-client dependency