osd: improve tests for configuration updates

Extract the default values from the actual configuration instead of
having them hardcoded. Also check that lowering the osd_map_cache_size
triggers the expected warning.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #2047 from dachary/wip-osd-config

osd: improve tests for configuration updates

Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>