Adding yamllinter to ci steps (#1556)

* Adding yaml linter to ci check

* Minor linting fixes from yamllint

* Changing CI to install python pkgs from requirements.txt

- adding in a secondary requirements.txt for tests
- moving yamllint to tests requirements

roles/network_plugin/weave/tasks/pre-upgrade.yml

@@ -1,3 +1,4 @@
+---
 - name: Weave pre-upgrade | Stop legacy weave
   command: weave stop
   failed_when: false