This change defines the contextmanager in_namespace[1]. It moves current
process in a network namespace (in __enter__) and moves it back in its
original network namespace (in _exit__) or kills current process if
__exit__ fails in order to ensure following commands will be executed
in the correct network namespace.
This change is an enabler to the Netlink solution to clean conntrack
entries.
[1] neutron_fwaas.privileged.utils
Partial-Bug: #1664294
Change-Id: I587257db8e1fce56a95f0db3dc4e0752751fdd81
094d2d5f01