For Ruby 2.7 tests use dedicated bundler version

Bundler 2.4.22 is last version to support Ruby 2.7 and needs to
be directly installed to prevent an error resolving the supported
version. This requirement occurred with the release of bundler 2.5.0.

theforeman.org/pipelines/lib/rvm.groovy

@@ -15,6 +15,11 @@ def gemset(name = null) {
 
 def configureRVM(ruby, name = '', bundler_version = null) {
     emptyGemset(ruby, name)
+
+    if (ruby == '2.7') {
+        bundler_version = '2.4.22'
+    }
+
     if (bundler_version) {
         withRVM(["gem install bundler -v '${bundler_version}' --no-document"], ruby, name)
     } else {