ganeti-rapi: Watch directory, not file for user file changes

We noticed several issues when just watching the file, among them race
conditions upon replacing the file using rename(2) (the new watcher
would be created too soon). By just watching the directory for events on
the rapi_users file, this can be avoided.

A nice side-effect is that now the users file is also reloaded if it
didn't exist upon ganeti-rapi's start (see the documentation update).

Since ganeti-rapi now becomes active for virtually every change in the
configuration directory (…/lib/ganeti), moving the rapi_users file to a
separate directory will be considered. It doesn't have to happen in or
before this patch, though.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>