Make "trusty" (Ubuntu 14.04) the default release

Ubuntu 14.04 was released today. Saucy will be EOL in 3 months.

Change-Id: I2043c3dfa8b795ae587b42cd80d3a0bf866bf82a
Small fixes for dhcp-all-interfaces

I3166951c6fbb2faec53ffc339edd2fd0c3512300 recently landed without two
fixes that I suggested, namely using rm -f rather than testing for
file existance, and a comment fix.

Change-Id: I68e187e20802f3482d4c524a6c435fec7526178e
Merge "Make "trusty" (Ubuntu 14.04) the default release"