Add support for mine.get in salt-ssh

Fixes #7903 

Added support for `mine.get` as a wrapper function.

Similar to publish, `mine.get` will call to each target using args from `mine_functions` out of the roster, pillar, or master config (in that order).

This is very slow, as it is basically a nested wfunc call, so we will generate pillar and grains a second time for each minion. However, it also means salt-ssh is mostly feature complete.

Will be adding to the mine docs tomorrow.