Add ability to enable/disable SSL

Instead of forcing people to use SSL (with invalid certificates, if they
don't have any valid ones), allow to not use SSL at all.

This is consistent with what is done in the other openstack barclamps.
Revert generate_certs to false

Now that SSL is disabled by default, we don't need to have this to have
a proposal that applies by default.
Merge pull request #187 from vuntz/ssl-enabled

Add ability to enable/disable SSL