Right now, `run_remote.go` only supports GCE instances. But actually
running the tests is completely independent of GCE and could work just
as well on any SSH-accessible machine.
This patch adds a new `--mode` switch, which defaults to `gce` for
backwards compatibility, but can be set to `ssh`. In that mode, the GCE
API is not used at all, and we simply connect to the hosts given via
`--hosts`.
This is still better than `run_local.go` because the latter mixes build
environment with test environment, which doesn't fit well with
container-optimized operating systems.
This is part of an effort to setup the e2e node tests on Fedora CoreOS
(see https://github.com/coreos/fedora-coreos-tracker/issues/990).
Patch best viewed with whitespace ignored.
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