Nova compute injects static interfaces file when DHCP ports belong to a different tenant than VM

[Pooja Ghumre]

Description
===========
When config drive is not used, metadata service is used to inject SSH key for which the static
route is injected by DHCP agent. When querying DHCP ports for a shared network, we need to use
admin context otherwise only for the network owner tenant, dhcp_server is added to network_info model of an instance and for other tenants, nova-compute injects static IP interface file.

Steps to reproduce
==================
* Created an isolated shared DHCO network in tenant A
* Created a VM on above network without config drive enabled and tried injecting SSH key
* Tried to SSH using the key

Expected result
===============
SSH using key should work.

Actual result
=============
Got permission denied error since SSH key was not injected.
Static route for metadata service was not added to guest and it had a static interfaces file injected instead of DHCP.

Environment
===========
- Openstack Newton (Nova and Neutron with metadata service enabled for isolated networks)