CI: Get job timeouts from environment

CI: Get job timeouts from environment

The job timeouts were hard coded in the jenkinsfiles which
means changes had to go through gerrit.  Now they are taken
from the following environment variables (and their defaults) that
can be set in Jenkins configuration...

TIMEOUT_GATES =      "60 MINUTES"
TIMEOUT_DAILIES =    "3 HOURS"
TIMEOUT_REF_DEBUG =  "24 HOURS"
TIMEOUT_UNITTESTS =  "30 MINUTES"

Change-Id: I673a551c1780bf665a3bc160b245da574aa4bbab