mgr/cephadm: mock CephadmHttpServer run in unit tests

mgr/cephadm: mock CephadmHttpServer run in unit tests

While tests were still passing, since this server class
was added the units tests are taking significantly longer to
run and I'm seeing messages like

Failed to run cephadm http server: Expected 4 octets in "<MagicMock name='mock.get().__getitem__()' id='139931011514704'>"

The run time given by "time tox -e py3" without this commit on
my local machine was

real	2m45.749s
user	16m13.273s
sys	0m11.041s

and with this commit (and the server run function therefore mocked)

real	0m36.164s
user	0m26.932s
sys	0m2.367s

so it seems to run about 4x faster with the server run function
mocked and the tests pass in both cases

Issue introduced by: https://github.com/ceph/ceph/pull/46400/commits/a0fe3ffdaaddd661f115de78ddae89b07b98a9ae

Signed-off-by: Adam King <adking@redhat.com>