Faucet

* Pin Travis docker to 18.03.1
* Fix duplicate broadcasts to mirror port, when mirroring multiple ports
* Test suite shouldn't assume all test switches have some hardware

* Ryu 4.26
* Fix BGP routes missing on VLAN reconfigursation
* Documentation updates for FAUCET on HPe 5400R series

* Use safe_load() for YAML (not load())
* Stacking config can use port names.
* Stacking hardware test should use configurable port range

* Add documentation for Cisco 9000 series switches
* FAUCET stacks can do automatic cabling verification if LLDP is enabled on stack links
* Fix FAUCET could send LLDP on non-stack links, when stacking was enabled
* Hardware test suite now supports some stacking tests

Workaround lack of availability of typing library on python3.4.

* Only provision flows for ports that are known to be up (fixes stacking when redundant links are down on initialization, among other cases)
* Test suite now drops traffic from host-side test interfaces when testing hardware switches (to stop DHCP, Avahi et al from contaminating the tests)
* Test suite can use external pip cache from host (faster test startup)