[FLOC-4541] Travis acceptance tests

[wallrj]

Fixes: https://clusterhq.atlassian.net/browse/FLOC-4541

Changes:

• run-acceptance-tests is called from travis -- via .travis/flocker_travis/script.py
• Created a small python package for the travis helper scripts so that I can share some functions between the script and after_script scripts.
• Added an artifact archiving step for uploading all the run-acceptance-tests log files to S3
• Added AWS acceptance.yml credentials and the private SSH key to the repo as an encrypted archive in .travis/secrets.tgz

TODO:

• Install the latest branch packages -- currently I'm installing the last released packages.
• Find a better S3 bucket for the artifacts -- I re-used clusterhq-dev-archive because the IAM user happened to have access to it.