[QA] salt master for CI instances

[Antoine Musso]

Hello,

To ease maintenance of Jenkins slaves in labs, we now have a salt master 
to easily run command on all instances.

The instance is integration-saltmaster.eqiad.wmflabs and you need to be 
root to use salt.

Example command:
   salt '*' cmd.run hostname

We have some very basic doc at:
   https://wikitech.wikimedia.org/wiki/Salt

Upstream documentation is known to be fairly complete:
   https://docs.saltstack.com/


The task was https://phabricator.wikimedia.org/T87819


-- 
Antoine "hashar" Musso