Revert "Salt Provisioner: refactor custom install_type option to add install_command instead"

Refers to issues #6276, #5973, #5936 and #5435

This reverts commit 72e63767ac.

Conflicts:
	website/docs/source/v2/provisioning/salt.html.md

plugins/provisioners/salt/provisioner.rb

@@ -136,20 +136,17 @@ module VagrantPlugins
         end
 
         if @config.install_type
-          options = "%s %s" % [options, @config.install_type]
+          # Allow passing install_args as an arbitrary string rather
+          # than trying to format it based on known options
+          if @config.install_type != "custom"
+            options = "%s %s" % [options, @config.install_type]
+          end
         end
 
         if @config.install_args
           options = "%s %s" % [options, @config.install_args]
         end
 
-        if @config.install_command
-          # If this is defined, we will ignore both install_type and
-          # install_args and use this instead. Every necessary command option
-          # will need to be specified by the user.
-          options = @config.install_command
-        end
-
         if @config.verbose
           @machine.env.ui.info "Using Bootstrap Options: %s" % options
         end

website/docs/source/v2/provisioning/salt.html.md

@@ -54,19 +54,14 @@ on this machine. Not supported on Windows guest machines.
 * `install_syndic`   (boolean) - Install the salt-syndic, default
 `false`. Not supported on Windows guest machines.
 
-* `install_type`  (stable | git | daily | testing) - Whether to install from a
+* `install_type`  (stable | git | daily | testing | custom) - Whether to install from a
 distribution's stable package manager, git tree-ish, daily ppa, or testing repository.
-Not supported on Windows guest machines.
 
 * `install_args` (develop) - When performing a git install,
 you can specify a branch, tag, or any treeish. If using the `custom` install type,
 you can also specify a different repository to install from.
 Not supported on Windows guest machines.
 
-* `install_command` (string) - Allow specifying an arbitrary string of arguments
-to the bootstrap script. This will completely ignore `install_type` and `install_args`
-to allow more flexibility with the bootstrap process.
-
 * `always_install`   (boolean) - Installs salt binaries even
  if they are already detected, default `false`