Vagrantfile: add a configuration patch to fix slow networking on some hosts.

Some hosts experience issues with networking (specifically, DNS latency) inside the development environment VM.

See also https://github.com/mitchellh/vagrant/issues/1172 and https://gist.github.com/simono/73e37d8c3a45664c7045 for a description of the problem and the origins of this fix.

This fixes issues with a number of integration tests which can fail on timeouts due to absurdly high (i.e. many seconds) DNS latency.  Another workaround that fixes the troublesome tests is `echo 192.0.2.100 dev.localflynn.com controller.dev.localflynn.com >> /etc/hosts`; this fix appears to get at the general purpose issue.  Not all hosts experience this issue; I don't know what the discriminating factor is, but I haven't seen any reports that this change could cause any detrumental behavior for those already unaffected.

(p.s. @lmars this is the stuff you suggested in IRC -- I just tested it, and afaict it works as advertised)