refactor: Retrieve method for descriptor in iptables plugin

[rewenset]

Noticed I've forgot to git add this changes yesterday. Comments fixing is the most important part in here :)

• Switch back to the default namespace asap.
  problem PR fixes:
  if d.ipTablesHandler.ListRules() returns error, call continue and skip switching to default network ns.
• Return early if no rule chain is present.
  problem PR fixes:
  check if if goRoutinesCnt == 0 and it could happen only if correlate is empty. Set it to 1 and call retrieveRuleChains anyway.

[codecov]

Codecov Report

Merging #1585 into dev will increase coverage by 3.88%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              dev   #1585      +/-   ##
=========================================
+ Coverage   50.21%   54.1%   +3.88%     
=========================================
  Files         673     370     -303     
  Lines       79720   31150   -48570     
=========================================
- Hits        40032   16853   -23179     
+ Misses      37165   12939   -24226     
+ Partials     2523    1358    -1165

Flag	Coverage Δ
#e2e	?
#unittests	54.1% <ø> (-0.01%)	⬇️