Valid login session length should be configurable

Currently, 3scale login session is fixed to 2 weeks and is not changeable. However, the length of the valid session should be configurable.

These session aspects should be customizable for the developer and for the admin portal:
1. Session Inactivity Timeout value

Implementation is to set `user_sesion_ttl` in settings.yml as seconds - something like 7200 (would mean 2 hours). In case setting value is not set (null) or is set to the empty string, then the fallback value of 2 weeks would be used.

This is the counterpart to THREESCALE-693 so the operator will allow passing this parameter down. Otherwise the customer has to mount the whole settings.yml and that is inconvenient, and potentially problematic when doing operator version upgrades.

Note: THREESCALE-693 implementation has been merged