Use project_reader in compute volumes tests

Add reader client setup for volumes_extensions_client and
snapshots_extensions_client to BaseV2ComputeTest.setup_clients()
in tempest/api/compute/base.py.

Update test files to use reader clients for read operations.

Change-Id: I24feaaced466e7539895b45ff35ebc10229acc73
Signed-off-by: Jason Paroly <jparoly@redhat.com>
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
6 files changed