test: Update flynn cli commands

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Support libvirt-lxc backend

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Build Flynn with 8 Cpus

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Use EC2 apt mirror when building rootfs

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Use 2GB RAM when building Flynn

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #79 from flynn/test-libvirt-backend

test: Support libvirt backend