Make obfsproxy optional

i have a vpn setup i'd like to deploy to. using the latest leap_platform:develop and leap_cli:develop, i get this:

Must pass bind_address to Class[Obfsproxy] at /srv/leap/puppet/modules/obfsproxy/manifests/init.pp:1 on node mail01.genopoly.org

either we need to make obfsproxy optional or update the documentation how to configure it right, if bind_address is really neccessary.

irregulator, i don't know if you're the right one to assign, if not, please reassign to elijah.

(from redmine: created on 2014-09-24, closed on 2014-09-25, relates #5714)