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.
Upgrade docker-client dependency
Fix error message in CLI

Fixes #264
Merge pull request #265 from spotify/dxia/fix-cli-err-msg

Fix error message in CLI
Use consistent jobPrefix for ProberTest

Use the same tag for jobs so that SystemTestBase can cleanup leftover
containers.

Related to commit 0a5b0f8e4563818541f4884e5c531ed89dcc4d6b

Fixes #263
Merge pull request #267 from spotify/dxia/cleanup-prober-test

Use consistent jobPrefix for ProberTest
Updating test to match recent change to CLI output
Merge pull request #268 from spotify/rculbertson/query-failure-test-fix

Updating test to match recent change to CLI output
Fix bug where JobStatusCommand can throw NPE

This could happen when getting host status. We first get a list of all
jobs, then the status for each job. If a job was deleted between those
two steps, we would throw a NPE when trying to process its status.

-RN
Fixed bug where 'helios status --host' would sometimes throw NPE
Merge pull request #266 from spotify/rculbertson/host-status-fix

Fix bug where JobStatusCommand can throw NPE