Needed because salt-cloud will attempt to load them even if they are
filled with invalid default values, creating error spam.

This obsoletes the salt:cloud:folders and salt:cloud:providers pillar
entries. Provider keys have been moved to /etc/salt/pki/cloud.

These aren't intended to function; they're here to allow the use of
file.recurse on the provider folder, without requiring the user
to provide pillar data for templates they're not using.

This is necessary for the correct settings on the minions when saltifying new hosts

Most include do not expect salt to be something else than the usual salt
variable giving access to all the salt modules. Instead we use cfg_salt.
And for consistency we rename the master/minion variables to
cfg_master/cfg_minion too.

This commit also provides a more concrete example of a 'host' to
be saltified.  Users can do

    salt-cloud -p make_salty someinstance

or

    salt-cloud -m /etc/salt/cloud.maps.d/foo.conf

Either which way the online docs should really be updated with more
concrete examples.