Use reader role for GET requests in flavors tests

Update all test files in tempest/api/compute/flavors to use the reader
role for GET requests (list, show operations) following the pattern
in test_list_server_filters.py. This ensures proper RBAC testing by
using the project_reader credentials for read operations while
maintaining the primary credentials for write operations.

Changes include:
- Added credentials = ['primary', 'project_reader'] to test classes
- Set up reader_client in setup_clients method
- Replaced all GET requests to use reader_client instead of regular client
- Updated files: test_flavors, test_flavors_negative

Partial-Implements: blueprint test-nova-with-srbac-defaults
Change-Id: I6fcc36342d2738aaa642539bef6501a28e7752ef
Signed-off-by: Jason Paroly <jparoly@redhat.com>
2 files changed