Default env vars for hosts/groups

[rollcat]

It should be possible to specify some default env var values per each host, or group of hosts.

Currently env vars can be specified on the command line for each run:

judo -e ANSWER=42 -c 'echo $ANSWER' localhost

However this doesn't scale well if different host groups need different values:

judo -e ANSWER=42 -c 'echo $ANSWER' hitchhikers
judo -e ANSWER=47 -c 'echo $ANSWER' trekkies

In addition to all the error-prone repetition, it fails to take advantage of concurrent execution between the two groups.

Perhaps we could use an envdir-like scheme, where per-host and/or per-group directories can reside under an env hierarchy:

env/
    hitchhikers/
        ANSWER
    trekkies/
        ANSWER

$ cat env/hitchhikers/ANSWER
42

This would also require for hosts to actually track which groups they belong to; there is groups []string in host definition but it's currently not populated. It's also a bit of a trap, since a host can only know that it's a part of a group, if the group has been explicitly targeted on the command line.