Add keychain and setup for jenkins user

We've been having issues with ssh agent forwarding and jenkins, which we
need to push data between our environments. Keychain is what GOV.UK uses
to manage the agent forwarding so we should use it too.
Move .bashrc config into deploy class

If we don't have a jenkins_key we don't really want to setup keychain so
move the inserting of .bashrc into the if in deploy.pp
Merge pull request #85 from alphagov/add-keychain-to-jenkins

Add keychain and setup for jenkins user