mkimage-debootstrap: set ubuntuLatestLTS as Trusty

Ubuntu 14.04 LTS (Trusty Tahr) was released on Thu, 17 Apr 2014; Update
ubuntuLatestLTS accordingly.

Docker-DCO-1.1-Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com> (github: jmccrohan)
Merge pull request #5642 from jmccrohan/master

Update mkimage-debootstrap "ubuntuLatestLTS" to "trusty"