Ref #534: Remove docker images on exit

[essobedo]

fixes #534

Motivation

The build runs out of disk space which prevents adding new integration tests

Modifications:

• Free up some disk space by:
  • following the workaround described in this ticket
  • removing pre-pulled docker images
• Add a new System property camel.karaf.itest.keep.docker.images to indicate whether the docker images should be removed after the test. Locally, you can leverage the Maven property keep.docker.images.on.exit to keep them or not. By default, the docker images are cleaned up to prevent the disk space leak
• Extend the forked process exit timeout to ensure that pax-exam can stop properly

Results

Without these changes, we had 21 Gb available for the build and 0 Gb available after.
With these changes, we have 35 Gb available for the build and 23 Gb available after.

[f2par0]

LGTM