Fix python::virtualenv to allow virtualenv to not require absolute path

Author: treydock

manifests/virtualenv.pp

@@ -49,7 +49,7 @@
   Integer                               $timeout         = 1800,
   String                                $pip_args        = '',
   String                                $extra_pip_args  = '',
-  Optional[Stdlib::Absolutepath]        $virtualenv      = undef,
+  Optional[String[1]]                   $virtualenv      = undef,
 ) {
   include python