cephadm/tests: add pyyaml dependency

cephadm/tests: add pyyaml dependency

node-proxy requires this dependency so it needs to be added as
dependency for tox testing.

Typical failure:

```
ImportError while importing test module '/root/ceph/src/cephadm/tests/test_agent.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib64/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_agent.py:10: in <module>
    _cephadm = import_cephadm()
tests/fixtures.py:14: in import_cephadm
    import cephadm as _cephadm
cephadm.py:32: in <module>
    from cephadmlib.node_proxy.main import NodeProxy
cephadmlib/node_proxy/main.py:2: in <module>
    from .redfishdellsystem import RedfishDellSystem
cephadmlib/node_proxy/redfishdellsystem.py:2: in <module>
    from .baseredfishsystem import BaseRedfishSystem
cephadmlib/node_proxy/baseredfishsystem.py:2: in <module>
    from .basesystem import BaseSystem
cephadmlib/node_proxy/basesystem.py:2: in <module>
    from .util import Config
cephadmlib/node_proxy/util.py:2: in <module>
    import yaml
E   ModuleNotFoundError: No module named 'yaml'
```

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit 6e7ea5172ac489c01e0a073acf869bcf6982a2b4)