stage0: error when using relative path for --exec

[crawford]

run: error parsing app image arguments: invalid argument "scripts/deploy stage" for --exec: --exec must be absolute path

I think it would be more accurate if it said argument to --exec must be an absolute path or 'scripts/deploy stage' is not an absolute path.

[jonboulle]

The middle part there is going to be a little trickier to change since it's part of pflag: https://github.com/coreos/rkt/blob/master/Godeps/_workspace/src/github.com/spf13/pflag/flag.go#L621

But this raises a different point which is that this check is actually wrong since appc/spec@7d3e414 landed