Fix the race condition in the image cap calculation

Use of a static synchronized hashmap `provisioningAmis` to keep track of the
number of containers per images being provisioned.
This code mainly comes from the jenkins ec2-plugin so all the credit goes to
its authors.
GH-28 Add a container cap by machine
Merge pull request #32 from dreewoo/master

Expose hostname flag to the UI
Merge pull request #34 from dreewoo/instance_cap

Add a per machine instance cap and fix instance cap limit calculation
Tone down the logging a touch (!). Use SLF4J @ debug level.

Try to make isAcceptingTasks() return more quickly, and avoid checking
the container in the cloud as this is subject to a timeout. If you have
a non-responsive host, it can seriously hold up the rendering of the
home page.

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>
[maven-release-plugin] prepare release docker-plugin-0.6.2
[maven-release-plugin] prepare for next development iteration
Turn down the logging a bit more

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>
When the SSH connection fails,
back off and retry.

Implement as a wrapper to the launcher, as this might be something that
ought to belong in the SSHSlaveLauncher.