enable gocritic linter

The linter ensures a common code style.
- use switch/case instead of else if
- use if instead of switch/case for single case statement
- add space between comment and text
- detect the use of defer with os.Exit()
- use short form var += "..." instead of var = var + "..."
- detect problems with append()
```
newSlice := append(orgSlice, val)
```
  This could lead to nasty bugs because the orgSlice will be changed in
  place if it has enough capacity too hold the new elements. Thus we
  newSlice might not be a copy.

Of course most of the changes are just cosmetic and do not cause any
logic errors but I think it is a good idea to enforce a common style.
This should help maintainability.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

pkg/namespaces/namespaces.go

@@ -375,7 +375,7 @@ func (n NetworkMode) Container() string {
 	return ""
 }
 
-//UserDefined indicates user-created network
+// UserDefined indicates user-created network
 func (n NetworkMode) UserDefined() string {
 	if n.IsUserDefined() {
 		return string(n)