[fullstack] Add option for custom apipaste.ini
[fullstack] Add option for custom apipaste.ini

networking-bagpipe's fullstack envrionment has several issues, one of
them is that neutron.tests.common.helpers.find_sample_file searches in
neutron root dir, and in case neutron is just installed in a virtenv,
like in case of bagpipe fullstack environment api-paste.ini is not in
place.

Related-Bug: #1859861
Change-Id: I2ba9e35998bbce45e76abb795ed6c242752c7f99

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron