Fix return type of get_app_port_mappings

[drewkerrigan]

This change addresses a bug introduced marathon-lb version 1.9.0 which affects apps using USER networking mode with containers as well as on DC/OS versions 1.9 and lower. Specifically apps with empty portMappings.

The encountered error looks similar to this:

File "/marathon-lb/utils.py", line 376, in get_port_mapping_ports
for p in port_mappings
TypeError: 'NoneType' object is not iterable

[nlsun]

we should remove the default and then have a specific check to return [] if None. This is because app is may have None as the value.

We partially mitigated this elsewhere with the "cleanup_json" function, but there is one other place in the code that gets json (marathon.list()) which we don't sanitize

[nlsun]

we can either try to sanitize all entrypoints of json (like marathon.list()) or have this function also handle a None aka json null

[drewkerrigan]

Please check #496 before we move forward with this PR

[drewkerrigan]

@nlsun @jdef Ready for review.

[drewkerrigan]

I've manually tested these fixes on DC/OS 1.9 and 1.10-beta2, merging and then releasing a new version.