Do not attach runner to a non-exsiting network in QA

qa/qa/service/runner.rb

@@ -15,6 +15,14 @@ module QA
         @tags = %w[qa test]
       end
+      def network
+        shell "docker network inspect #{@network}"
+      rescue CommandError
+        'bridge'
+      else
+        @network
+      end
+
       def pull
         shell "docker pull #{@image}"
       end
@@ -22,7 +30,7 @@ module QA
       def register!
         shell <<~CMD.tr("\n", ' ')
           docker run -d --rm --entrypoint=/bin/sh
-          --network #{@network} --name #{@name}
+          --network #{network} --name #{@name}
           -e CI_SERVER_URL=#{@address}
           -e REGISTER_NON_INTERACTIVE=true
           -e REGISTRATION_TOKEN=#{@token}

qa/qa/service/shellout.rb

@@ -3,6 +3,8 @@ require 'open3'
 module QA
   module Service
     module Shellout
+      CommandError = Class.new(StandardError)
+
       ##
       # TODO, make it possible to use generic QA framework classes
       # as a library - gitlab-org/gitlab-qa#94
@@ -14,7 +16,7 @@ module QA
           out.each { |line| puts line }
           if wait.value.exited? && wait.value.exitstatus.nonzero?
-            raise "Command `#{command}` failed!"
+            raise CommandError, "Command `#{command}` failed!"
           end
         end
       end