When multiple processes are spawned with the same configuration
each process has the same backdoor_socket path configured and
only the first process able to bind to the socket can later be
accessed via the backdoor. To give each process a unique socket path
we now expose the PID of the process as a format string argument,
which can then be used like this:
backdoor_socket = /var/lib/neutron/backdoor-{pid}
Change-Id: I3f86f4867eb0cd5010abadf68620aa3450d3e64d
dd174fbfde