Don't assume nsenter and docker-enter are in the same directory

In coreos, `nsenter` is built-in and installed in the read-only
partition as `/usr/bin/nsenter`.
A systemd service must not installs docker-enter in `/usr/bin`.

This change makes sure that we force the full path of nsenter if and
only if it exists in the same directory.
I am not sure why the full path was needed in the first place:
it works without it in coreos.