core: only check for box existence if not force [GH-2757]

CHANGELOG.md

@@ -45,6 +45,7 @@ BUG FIXES:
     syntax errors.
   - core: Plugins that fork within certain actions will no longer hang
     indefinitely. [GH-2756]
+  - commands/box: Box add `--force` works with `--provider` flag. [GH-2757]
   - commands/plugin: Plugin installation will fail if dependencies conflict,
     rather than at runtime.
   - guests/redhat: Set hostname to FQDN, per the documentation for RedHat.

lib/vagrant/action/builtin/box_add.rb

@@ -33,7 +33,7 @@ module Vagrant
 
           # Determine if we already have the box before downloading
           # it again. We can only do this if we specify a format
-          if box_formats
+          if box_formats && !env[:box_force]
             begin
               if env[:box_collection].find(box_name, box_formats)
                 raise Errors::BoxAlreadyExists,