Replace JENKINS_URL with JENKINS_PUBLIC_URL

To ensure tf-a-ci-scripts can run on both OpenCI
and Arm Test CI, JENKINS_URL has been replaced with
JENKINS_PUBLIC_URL, as JENKINS_URL is a reserved
private URL in Arm infrastructure.

JENKINS_PUBLIC_URL is a system-wide environment
variable and was configured in:
Manage Jenkins > System > Global Properties

Default values:
OpenCI Production Jenkins: https://ci.trustedfirmware.org/
OpenCI Staging Jenkins: https://ci.staging.trustedfirmware.org/
Arm test CI: https://jenkins.openci-test.arm.com/

Change-Id: I59264f434fc162b5a8122688a0102269b3cb792c
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
10 files changed