Add http-service, a REST API for running tests

and getting the current status of the system.
Fix HTTP service tests

I'm not particularly happy about this.  It doesn't quite seem like the
right place for it but we are expecting some fairly serious refactoring
of the http-service and this makes the tests pass so I'll leave it in
for now.
ssh-service run: Allow using a local repository

The http-service will typically use a local repository as git source, but
the `stbt-ssh-service run` doesn't allow the test-pack to be local as
this could be a security hole.

With this commit setting the `STBT_LOCAL_TEST_PACK` environment variable
pokes a hole in this verification specifically for the local test-pack
repo.  This is still secure as only the http-service has control over
`run`'s environment, but the user over http and ssh doesn't.

It's still not that pretty though.
http-service: Cosmetic tidy-up
fixup! ssh-service run: Allow using a local repository
http-service: Write logs out to stdout when calling `run`

This makes problems easier to debug as they will be printed as a part
of `make check` output.  I'm not sure why I didn't have this like this
before.