Bugfix: remove double quote around SECURE variable

Bugfix: remove double quote around SECURE variable

JIRA: YARDSTICK-689

This patch remove the double quote around the SECURE varibale as it may cause the fowllowing error:
$ openstack "${SECURE}" image list
openstack: ' image list' is not an openstack command. See 'openstack --help'.
Did you mean one of these?
  access token create
  address scope create
  address scope delete
  address scope list
  address scope set
  address scope show
  aggregate add host
  aggregate create
  ...

Change-Id: Ibb22e85b3dd89b0e3b62821bd5ebe4f155886ffc
Signed-off-by: JingLu5 <lvjing5@huawei.com>