ganeti.ssconf: Add function to write ssconf files

This function will be used to write ssconf files from the node daemon.
By creating a lock file, we synchronize different child processes of
ganeti-noded to not overwrite each other's changes. Also, external
processes could use this lock to make sure they don't read ssconf
files while changes are going on.

The socket module is not used (anymore) in ganeti.ssconf, removing it.

Reviewed-by: iustinp