ISO signing via a remote server may fail due to network hiccups. Retry
on errors.
TESTS
======================================
1) Run build-iso Jenkins job normally and make sure it works
2) Run build-iso with SIGNING_SERVER set to an invalid host name and
make sure it retries
Story: 2010226
Task: 48064
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>
Change-Id: Icaa8e07827ddfcc2583f875e5a57247ce7bf8613