Add time_to_live parameter into the barclamp

Add the parameter time_to_live (ceilometer.conf) into the barclamp,
so the user can decide when the meter lines are removed from the
database.

This patch was tested with a mongodb configuration.

This patch was made by Jiri (jsuchome). I only tested it and added
the parameter in the ceilometer.conf.erb template.
I think that using namespaces makes sense is we wan to scale the code in the future, but is true that is not consistent with some other barclamps (including this one)

I will fix both issues that @tboerger mention using this 'database' level, and if we finally decide to use a plain namespace, I will do so too.