Define Jenkins via set of ansible roles

Add these role submodules:

- ansible-slr-jenkins-base
- ansible-jenkins
- ansible-rbenv
- ansible-nodejs

The Jenkins box is configured as both master & slave, having a number of
plugins automatically installed and dependencies such as rbenv, s3cmd,
npm, grunt.
Merge pull request #2 from sealevelresearch/jenkins-ansible

Define Jenkins via set of ansible roles