mgr/tox: run pytest in parallel

mgr/tox: run pytest in parallel

The py3 and coverage tox environments run the full mgr pytest suite
serially, which makes run-tox-mgr the longest test in CI. Add
pytest-xdist and pass `-n auto` to both so the suite is distributed
across the available CPUs.

pytest-xdist is constrained to <2 to stay compatible with the pinned
pytest-cov. Running in parallel also surfaced a hard-coded port in
cephadm's test_node_proxy, which now allocates an ephemeral port per
process.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>