Add a new env var for nameserver

Add a new env var for nameserver

We should still consider that the nameserver must be handled by all
installers instead [1]:
  - by configuring one or more DNS resolvers in dhcp_agent.ini
  - by asking a local resolv

It eases setting the nameserver in testcases and then allows gating
them. 8.8.8.8 is still defined by default.

[1] https://docs.openstack.org/neutron/pike/admin/config-dns-res.html

Co-Authored-By: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: Id58dd5ebee612a0fce2ebc40f089769e60899416
Signed-off-by: Linda Wang <wangwulin@huawei.com>
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>