CCF node to write its pid to file

[olgavrou]

Regarding this discussion here
it would be nice to be able to tell the CCF nodes to write their process id to a file on startup, e.g. by passing a --pidfile argument

[jumaffre]

You can already retrieve the pid from the sealed secrets file created by each node when starting/joining a network. E.g. sealed_secrets.20191105114613.22017 (22017 is pid).

But of course, this file will go when we have implemented member key shares.

[achamayou]

Finding that file is slightly involved though, it'd be better for the remotes (and for the average daemonisation tool) to have node.pid somewhere deterministic.