Add advsvc role to neutron policy file

Add advsvc role to neutron policy file

Add in a default "advsvc" user and the logic in the Neutron policy
infrastructure which will allow this user to create/get/update/delete
ports on other tenants networks, as well as view other tenants
networks. This is for the use case of letting advanced services have
a user to put ports on other tenants networks. By default, we do not
define any roles for the policy "context_is_advsvc", but rely on
operators to specify the likely value of "role advsvc".

DocImpact

Closes-Bug: #1331836

Change-Id: I94cb3383eb1fed793934719603f888dbbdbbd85a
Co-Authored-By: Susanne Balle <sleipnir012@gmail.com>