cephadm: increase default retry_max value

cephadm: increase default retry_max value

When playing with cephadm, at multiple times, I've reached the max
number of attempt in `is_available()`

Increasing the `retry_max` helps to avoid failure like following:

```
INFO:cephadm:mgr not available, waiting (1/5)...
INFO:cephadm:mgr not available, waiting (2/5)...
INFO:cephadm:mgr not available, waiting (3/5)...
INFO:cephadm:mgr not available, waiting (4/5)...
INFO:cephadm:mgr not available, waiting (5/5)...
ERROR: mgr not available after 5 tries

```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>