get_root_helper_child_pid recursively finds the child of pid,
until it can no longer find a child. However, the intention is
not to find the deepest child, but to strip away root helpers.
For example 'sudo neutron-rootwrap x' is supposed to find the
pid of x. However, in cases 'x' spawned quick lived children of
its own (For example: ip / brctl / ovs invocations),
get_root_helper_child_pid returned those pids if called in
the wrong time.
Change-Id: I582aa5c931c8bfe57f49df6899445698270bb33e
Closes-Bug: #1558819
fd93e19f2a