TST: bootstrap chroot in tmpfs

during bootstrap eatmydata is not available in the chroot so it takes a
significant time. Avoid this by placing the whole chroot in a tmpfs
ramdisk.
TST: switch i386 build to python3.4 defaulting ubuntu 14.04

to do that fix a missing $DIST variable usage in the test script
Merge pull request #4389 from juliantaylor/test-backports-1.8

Test backports 1.8