Add bindep_command and bindep_fallback to site-variables
The bindep role is set up to allow setting a bindep command and fallback
file via variables, and to do the right thing if they're not there.
Add bindep_command and bindep_fallback to our site-variables so that we
can remove copy-pasta references from jobs.
# This file contains ansible variables that are used by our zuul-executors.
---
zuul_site_mirror_fqdn: "mirror.{{ nodepool.region | lower }}.{{ nodepool.cloud | lower }}.openstack.org"
+bindep_command: /usr/bindep-env/bin/bindep
+bindep_fallback: /usr/local/jenkins/common_data/bindep-fallback.txt
