docker client pool leak mitigation

Create and tear down the client at each usage.

Back Story: we've seen issues where excessive communication with the docker daemon has left the agent in a stuck state. Upon investigation, it appears that threads are stuck attempting to get connections from the connection pool in the docker client. We dug in some more and cannot figure out why, given the circumstances we see, how the pool would be exhausted. So to avoid the whole thing, we just create clients when we need them, and dispose of them right away.
Merge pull request #300 from spotify/dxia/upgrade-docker-client

Upgrade docker-client dep to 2.7.1
Add an end-to-end test for syslog redirection

1. Make the path to syslog used by SyslogRedirectingContainerDecorator
   configurable via the SYSLOG_REDIRECTOR job environment variable.

2. With an image that has a prespecified ENTRYPOINT and CMD, test that
   redirection actually works is sending stdout to the specified endpoint,
   while respecting the command specified in the Dockerfile.
Merge pull request #301 from spotify/rohan/syslog

Add an end-to-end test for syslog redirection
Merge pull request #297 from spotify/drewc/dockerpoolleak

docker client pool leak mitigation
Fix NPE
Merge pull request #302 from spotify/drewc/npefix

Fix NPE
Undo leak mitigation as it causes test failures

failures are due to weirdness in stuff underlying docker client
Merge pull request #303 from spotify/drewc/backoutleakmitigation

Undo leak mitigation as it causes test failures
Set longer timeout when waiting for sonatype

We've gotten timeouts when waiting for sonatype to perform
the release operation. Let's change it to 10 minutes from
the default of 5 minutes. Also bump to the latest version.
Fix NPE thrown by Optional

Need to call 'fromNullable' instead of 'of'
Merge pull request #306 from spotify/rculbertson/npe

Fix NPE thrown by Optional
Merge pull request #305 from spotify/rculbertson/nexus-timeout

Set longer timeout when waiting for sonatype
Fix SyslogRedirectionTest

Wait for container to exit to ensure cmd was run
and logs aren't empty which causes the test to fail.
Merge pull request #307 from spotify/dxia/fix-SyslogRedirectionTes

Fix SyslogRedirectionTest
Test that we can deploy a large number of jobs

Deploy 40 jobs, kill all the underlying containers, and make sure the
system recovers.
Merge pull request #308 from spotify/rohan/concurrent

Test that we can deploy a large number of jobs
Improve helios host output message

Clarify a comment and remove a TODO

Fixes #304
Merge pull request #309 from spotify/dxia/better-host-msg

Improve helios host output message
Upgrade docker-client dep to 2.7.3