Use ansible-jenkins roles

Not only takes care of installing Jenkins but already
does have a parameter to specify which plugins need
to be installed.

Removed everything on our role that is already taken care
with ansible-jenkins.
Configure security

Use Github Authentication Plugin to authenticate users.

Authorize through Github Commiter Authorization Strategy.

This basically means that github takes care of authenticating
and on jenkins itself we decide who can make changes and see
jenkins status.

Note that a github application has been created and its id and
secret data are here, so don't use it in production without first
modifying those!
Install dependencies needed to run jobs
Merge pull request #8 from plone/ansible-jenkins-dependency

Ansible jenkins dependency