Add IpFilter, IPNetnsExecFilter and EnvFilter

These filters have been implemented in Quantum before:

- IpFilter provides support for filtering ip commands
- IpNetnsExecFilter is a chaining command filter that
  verifies that the command to be executed by ip netns exec
  is covered by other established filters.

IpNetnsExecFilter has been restricted to ensure that the
filter chains have all matching filters run as the same
user.

EnvFilter is a new filter derived from CommandFilter
that allows a Command to be optionally prefixed by "env"
and a specific list of environment variables.

This is intended to replace the specific DnsmasqFilter
and DnsmasqNetnsFilter in the future when all consumers
have been updated.

Implements bp rootwrap-quantum-features

Change-Id: I0cf39967126e99a8dc53d21bee824a0fe2f63aa0
Merge "Add IpFilter, IPNetnsExecFilter and EnvFilter"