Change hard coded series fallback for LatestLtsSeries() to trusty

Ported from 1.20

Fixes:
https://bugs.launchpad.net/bugs/1315473
https://bugs.launchpad.net/bugs/1363971

The LatestLtsSeries() detection had a hard coded fallback to "precise" when the call to 'distro-info --lts' fails. The fallback is changed to "trusty".

This covers the add-machine case (whether adding a container or a new machine) where the default-series config value is not specified.