Refactor functions.sh script

This change refactors some common instructions used by the functions.sh
script.

Signed-off-by: Victor Morales <v.morales@samsung.com>
Change-Id: Ic8cf3cfdd4a46d6fc32f45e4021a07cadc027736
Reviewed-on: https://gerrit.opnfv.org/gerrit/c/kuberef/+/71378
Tested-by: jenkins-ci <jenkins-opnfv-ci@opnfv.org>
Reviewed-by: Georg Kunz <georg.kunz@ericsson.com>
Reviewed-by: Michael Pedersen <michaelx.pedersen@intel.com>
Reviewed-by: Rihab Banday <rihab.banday@ericsson.com>