upnp: T4613: Verify listen key in dictionary

[sever-sever]

Change Summary

There is no check if listen exists in the dictionary, fix it
Fix odd ValueHelp format
Delete unused imports

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4613

Component(s) name

upnp

Proposed changes

How to test

Before fix:

set service upnp rule 10 action 'allow'
set service upnp rule 10 external-port-range '1024-65535'
set service upnp rule 10 internal-port-range '1024-65535'
set service upnp rule 10 ip '10.0.0.1'
set service upnp wan-interface 'eth0'

commit
Traceback (most recent call last):
  File "/usr/libexec/vyos/conf_mode/service_upnp.py", line 152, in <module>
    verify(c)
  File "/usr/libexec/vyos/conf_mode/service_upnp.py", line 116, in verify
    for listen_if_or_addr in upnpd['listen']:
KeyError: 'listen'



[[service upnp]] failed

After fix:

vyos@r1# commit
[ service upnp ]
Listen address or interface is required!

[[service upnp]] failed
Commit failed
[edit]
vyos@r1# 

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly