* network_plugin/custom_cni: add CNI to apply provided manifests

Add a new simple custom_cni to install provided Kubernetes manifests.
This could be useful to use manifests directly provided by a CNI when
there are not support by Kubespray (i.e.: helm chart or any other manifests
generation method).

Co-authored-by: James Landrein <james.landrein@proton.ch>
Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* network_plugin/custom_cni: add test with cilium

Co-authored-by: James Landrein <james.landrein@proton.ch>
Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

---------

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>
Co-authored-by: James Landrein <james.landrein@proton.ch>

* 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