fix empty listener descriptions (#142)
fix empty listener descriptions (#142)

* fix empty listener descriptions

backtrace was:
```
Traceback (most recent call last):
  File "/usr/local/bin/zmon-aws-agent", line 11, in <module>
    load_entry_point('zmon-aws-agent==0.2', 'console_scripts', 'zmon-aws-agent')()
  File "/usr/local/lib/python3.6/dist-packages/zmon_aws_agent-0.2-py3.6.egg/zmon_aws_agent/main.py", line 210, in main
  File "/usr/local/lib/python3.6/dist-packages/zmon_aws_agent-0.2-py3.6.egg/zmon_aws_agent/aws.py", line 332, in get_running_elbs
  File "/src/opentracing-utils/opentracing_utils/decorators.py", line 61, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/zmon_aws_agent-0.2-py3.6.egg/zmon_aws_agent/aws.py", line 365, in get_running_elbs_classic
IndexError: list index out of range
```

* use dnsname only when there is no protocol

* fix extra ,

zalando/zmon-aws-agent,zalando/zmon-aws-agent