Kubernetes connection support for mitogen.

[Yannig]

This PR implements a kubernetes connector for mitogen.

You can test this PR using a running minikube and by running the following commands:

$ export ANSIBLE_CALLBACK_WHITELIST=profile_tasks,timer

$ ansible-playbook tests/ansible/test-kubectl.yml

Here the result of this run:

PLAY [Create pod] **********************************************************

TASK [Create a test pod] ***************************************************
Wednesday 19 September 2018  16:50:21 +0200 (0:00:00.032)       0:00:00.032 *** 
ok: [localhost] => (item=0)
ok: [localhost] => (item=1)
ok: [localhost] => (item=2)
ok: [localhost] => (item=3)
ok: [localhost] => (item=4)
ok: [localhost] => (item=5)
ok: [localhost] => (item=6)
ok: [localhost] => (item=7)
ok: [localhost] => (item=8)
ok: [localhost] => (item=9)

[...]

PLAY [Destroy pod] *********************************************************

PLAY RECAP *****************************************************************
localhost                  : ok=3    changed=0    unreachable=0    failed=0   
test-pod-0                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-1                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-2                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-3                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-4                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-5                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-6                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-7                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-8                 : ok=4    changed=2    unreachable=0    failed=0   
test-pod-9                 : ok=4    changed=2    unreachable=0    failed=0   

Wednesday 19 September 2018  16:51:16 +0200 (0:00:00.361)       0:00:55.149 *** 
=============================================================================== 
Simple shell with linear ----------------------------------------------- 22.06s
Simple file with linear ------------------------------------------------ 18.36s
Create a test pod ------------------------------------------------------- 5.97s
Simple shell with mitogen ----------------------------------------------- 5.61s
Wait pod to be running -------------------------------------------------- 2.61s
Simple file with mitogen ------------------------------------------------ 0.36s
Add pod to pods group --------------------------------------------------- 0.15s

Shell is about 4 times faster and simple file verification is about 40 times faster!

[dw]

I guess now I'll need a VM with kubectl to play with :P

Don't worry about CI, there are a ton of known issues at present unrelated to this PR that will be fixed before 0.2.3 is released. Thanks for contributing this!

[Yannig]

Great!
To get a Kubernetes up and running in a minute, you can use minikube. It works for any OS of the market.
https://github.com/kubernetes/minikube