nat: T4499: Fix NAT not showing a single flow entry

[sever-sever]

Change Summary

We must change the dictionary if we get only one flow entry. Op-mode
I.e one NAT record
With single entry we get:

    OrderedDict([('meta', xxx]))

We expect:

    [OrderedDict([('meta', xxx]))]

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/T4499

Component(s) name

nat

Proposed changes

How to test

Get only one NAT flow:

vyos@r14:~$ sudo conntrack -L --src-nat
icmp     1 29 src=192.0.2.10 dst=1.1.1.1 type=8 code=0 id=2156 src=1.1.1.1 dst=192.168.122.14 type=0 code=0 id=2156 mark=0 use=1
conntrack v1.4.6 (conntrack-tools): 1 flow entries have been shown.
vyos@r14:~$ 

Before fix we don't see any data:

vyos@r14:~$ show nat source translations
Pre-NAT              Post-NAT             Prot  Timeout  
vyos@r14:~$ 
vyos@r14:~$ show nat source translations detail 
Pre-NAT src          Pre-NAT dst        Post-NAT src         Post-NAT dst      
vyos@r14:~$ 

After fix:

vyos@r14:~$ show nat source translations 
Pre-NAT              Post-NAT             Prot  Timeout  
192.0.2.10           192.168.122.14       icmp  29       
vyos@r14:~$ 
vyos@r14:~$ 
vyos@r14:~$ show nat source translations detail 
Pre-NAT src          Pre-NAT dst        Post-NAT src         Post-NAT dst      
192.0.2.10           1.1.1.1            192.168.122.14       1.1.1.1           
  icmp: 192.0.2.10 ==> 192.168.122.14 timeout: 29 use: 1 
vyos@r14:~$ 

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