test: skip docker tests if docker is misconfigured

Rather than assume sudo access, the test scripts assume docker has been
configured to allow non-root access.
test: fix systemd service installation test

The node in PATH may be a symlink, in which case node's
process.execPath (which is what is used by the installer) won't be the
same so the test will always fail.
Merge pull request #77 from strongloop/docker-and-systemd-test-cleanup

Docker and systemd test cleanup