feat: get swagger instance from register

[rytsh]

Describe the PR
Problem starts if our swagger instance generated by CI and registered.
In main program if we want to modify some parameters inside of that instance is it not possible because code not in there.

Getting swagger instance with name from register it is easy to work with it, without need any workaround.

Swagger is an interface so we should do some casting but we know the our code and it should be this type.

if s, ok := swag.GetSwagger("swagger").(*swag.Spec); s != nil && ok {
	s.Version = "1.0.0"
	s.Title = "Test"
}

Relation issue
#1240

[ubogdan]

LGTM

[codecov]

Codecov Report

Merging #1298 (11143e9) into master (23661ef) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1298   +/-   ##
=======================================
  Coverage   95.04%   95.05%           
=======================================
  Files          14       14           
  Lines        2787     2791    +4     
=======================================
+ Hits         2649     2653    +4     
  Misses         79       79           
  Partials       59       59           

Impacted Files	Coverage Δ
swagger.go	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[ubogdan]

@rytsh Thanks for your contribution.