Cleanup process manager get_pid_file_name

This function name is "get pid file name" but it will actually
create the directory in some cases, that's broken down to
the utils module.

I propose to separate into two functions, and change callers
to do "ensure directory" via different method, instead of passing
the ensure_directory flag.

See discussion here: https://review.openstack.org/#/c/115935/20/neutron/agent/linux/external_process.py