Add non-configurable attribute for http/https use in keystone

In short: this doesn't change the current behavior, and is only a
preparatory commit for future SSL support in keystone.

This is set to http right now; this will enable other barclamps to use
this attribute without breaking them, and then we can make this
configurable.
Merge pull request #81 from vuntz/protocol-attribute

Add non-configurable attribute for http/https use in keystone [1/7]