Allow some of the default directives in modules/cfg.py to be overridden from ravshello config file

Specifically, these:

```
defaultAppExpireTime = 120
defaultAppExtendTime = 60

maxLearnerPublishedApps = 3
maxLearnerActiveVms = 8
maxLearnerExtendTime = 120
```

Should also add a new `ravelloCacheTimeout` setting and use that in `ravello_cache.RavelloCache` instead of the hard-coded 120-sec timeouts.