Check nsenter before forcing full path to it

`nsenter` is not necessarily in the same folder than `docker-enter`.

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.