get_run_path: create path if we're making one up

Otherwise we get:

Traceback (most recent call last):
  File "./lxc", line 91, in <module>
    args.func(args)
  File "/home/ubuntu/lxc/src/lxccmd/lxccmd/commands/server/__init__.py", line 216, in cli_start
    with open(os.path.join(get_run_path(), "server.pid"), "w+") as fd:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/lxc-1000/server.pid'

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>