Porting function_exists to post_test_hook.sh
Porting function_exists to post_test_hook.sh

Porting function_exists to post_test_hook.sh from devstack

Change-Id: I8341ccfd254fde42d1ce7c4b2e157f4d1a3b7102
Closes-Bug: #1493761

openstack/magnum,Tennyson53/magnum,mjbrewer/testindex,jay-lau/magnum,openstack/magnum,mjbrewer/testindex,dimtruck/magnum,mjbrewer/testindex,dimtruck/magnum,Tennyson53/magnum,eshijia/magnum,ArchiFleKs/magnum,eshijia/magnum,ArchiFleKs/magnum