stage4: Update to use go 1.6, update to stage3 with glibc-2.21-r2
stage4: Update to use go 1.6, update to stage3 with glibc-2.21-r2

This removes the upgrade to glibc-2.21-r2. The package was unmasked
upstream and this update includes pulling in a new stage3 image which
already includes the package.

This also moves from go 1.5.3 to go 1.6 as the build version.

apcera/kurmaos,apcera/kurmaos,apcera/kurmaos,apcera/kurmaos