Remove possibility to use sqlite

It's known to be broken, and there was agreement to only use the
database barclamp.

This means we can remove the whole database_engine dance, which
simplifies things.
Use new protocol attribute from keystone barclamp
Merge pull request #89 from vuntz/no-sqlite

Remove possibility to use sqlite
Merge pull request #91 from vuntz/keystone-protocol-attribute

Add non-configurable attribute for http/https use in keystone [2/7]