push_to_asf.py: Use check_output() from kudu_util
push_to_asf.py: Use check_output() from kudu_util

This makes the script compatible with older versions of python again.

Change-Id: I75570c408f63bc5a0a4023e9d50b888c43eca16e
Reviewed-on: http://gerrit.cloudera.org:8080/5469
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@apache.org>
Tested-by: Kudu Jenkins

EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu