Use project_reader in compute floating_ips tests

We have project_reader credentials in tempest and they
use the same project as member role creds. Nova support
the project reader role in get server API. Moving the
compute floating_ip  tests to use project reader creds in GET
API calls for RBAC support.
Changes include:
- Set up reader_floating_ips_client in base class setup_clients method
- Replaced all GET requests to use reader_servers_client instead of
  regular client

Partial-Implements: https://blueprints.launchpad.net/tempest/+spec/test-nova-with-srbac-defaults

Change-Id: I4f6f2597d096d4e4b35edd7090a8e45c5d78d340
Signed-off-by: Ashish Gupta <ashigupt@redhat.com>
5 files changed