In source images, keystone-manage is installed to a virtualenv in
/var/lib/kolla/venv. This is not in the PATH for cron jobs, which always
use PATH=/usr/bin:/bin. This results in the following error:

/usr/bin/fernet-rotate.sh: line 3: keystone-manage: command not found

However this error is not typically visible, since cron logs to syslog
and we do not configure fluentd to collect these logs.

This change configures the PATH in the fernet-rotate.sh script for
source images.

Change-Id: Ib49ea586d36ae32d01b9610a48b13798db4a4cd5
Closes-Bug: #1850711