Remove dangerous arguments default

Method manila.network.linux.ip_lib.IpNetnsCommand.execute use mutable
object as default value for argument addl_env. In 99% cases this lead to
incorrect results and it is too difficult to track what is the source of
the problem.

You never should use mutable default value for function/method argument.
Unless you really know what are you doing.

Change-Id: Iba93e55f2ead5bfc0fd4986d49f872783cf64d99
Merge "Remove dangerous arguments default"