fix(set_lsb_release): Add option for production updates, misc updates.

Add --production_track argument to set_lsb_release and
cros_make_image_bootable to support using the production update service
on developer builds of the 'prod' image. This replaces the previous hack
of setting COREOS_OFFICIAL=1 in the middle of the build.

Since lsb-release doesn't exist prior to the first call to
set_lsb_release switch to sudo_clobber instead of append. That way if it
is called a second time later the contents aren't duplicated.

Write the info to gentoo-release and os-release as well so everything
gets the same information.
Merge pull request #57 from marineam/lsb_release

fix(set_lsb_release): Add option for production updates, misc updates.