Support per-backend options

varnish_backend_options can now be a list of hashes, with the
'servers' key in each hash matching against the server's FQDN
to specify for which backends the options apply.

The old style single hash for all backends is also still
supported.

Change-Id: I939fc01054375536c5fca6f169e45be629aa51ce
Merge "Support per-backend options" into production