Run ansible in a crontask on service hosts

Because this won't work well in development (it will always look at
prod-hosts), it's easily disabled with no_ansible_pull=true in the
inventory.  Fixes #3089.
use a git:// URL
Focus on local.yml for all Ansible runs
configure service hosts too, and move ansible-pull tasks to 'base'
install ansible repo in service account's home
mention ansible runs non-priviledged
Fix bootstrapping to work with local.yml.
use an explicit branch
fix README typo
Merge djmitche/buildbot-infra:pull-cron (PR #37)