Varnish trigger improvements:

- Honour the varnish timeout configuration
- Honour the varnish critical configuration
- Honour the varnish retry configuration
- Cleanup the reconnect-on-fetch-error handling

See https://github.com/Vizzuality/cartodb-management/issues/1129

Tested with Varnish-Cache 2.1.5 and python-varnish 0.1.2 with
varnish-management.critical set to both true and false.