Remove PROJECT_ADMIN limitation from zero-disk and external-network policy
Remove PROJECT_ADMIN limitation from zero-disk and external-network policy

In Ussuri, we added the PROJECT_ADMIN default policy for POST /servers
API in case of 1. forced_host 2. requested_destination 3. zero_disk_flavor
4. network_attach_external [1]. For 1st two we have the limitation of
project_admin to get the host name and pass it in POST /servers request.

But for last two (3. zero_disk_flavor, 4. network_attach_external) we do
not have such limitation:

3. zero disk flavor - This policy is checked to protect from the large image
 and indicating the server should be volume-backed.
 - https://github.com/openstack/nova/blob/c0c2888acaaccfa4266874523f62039cb9e143e0/nova/compute/api.py#L751

4. Attach an unshared external  - It depends on neutron policy for
 get external network. If user want to create server with net id then
 they can get net id from neutron because neutron policy for GET external
 network is SYSTEM_OR_PROJECT_READER[2]. Otherwise requested projects
 (who is creating server) networks will be fetched from neutron[3]. so
 with neutron default policy there is no limitation here.

[1]
https://github.com/openstack/nova/blob/cd084aeeb8a2110759912c1b529917a9d3aac555/nova/policies/servers.py#L189-L217
https://github.com/openstack/nova/blob/cd084aeeb8a2110759912c1b529917a9d3aac555/nova/policies/servers.py#L279-L314
[2] https://github.com/openstack/neutron/blob/0bdf3b56e0d4ede2d46eed09a4bb07dd3c00807d/neutron/conf>
[3] https://github.com/openstack/nova/blob/7cabd6dc40196aada7e9dba9382b8ae0c2a4bdb6/nova/network/ne>

Change-Id: Ibf45c02fae6f6b0b39dc4de206416f03c801351b

mahak/nova,openstack/nova,mahak/nova,openstack/nova,openstack/nova,mahak/nova