core: Allow strings to be used when checking for Guest capabilities

CHANGELOG.md

@@ -7,6 +7,7 @@ IMPROVEMENTS:
 
 BUG FIXES:
 
+  - core: `Guest#capability?` now works with strings as well
   - core: Fix NoMethodError in the new `Vagrant.has_plugin?` method [GH-2189]
   - hosts/arch: NFS exporting works properly, no exceptions. [GH-2161]
   - hosts/fedora: Fix host detection encoding issues. [GH-1977]

lib/vagrant/guest.rb

@@ -113,7 +113,7 @@ module Vagrant
     #
     # @return [Boolean]
     def capability?(cap_name)
-      !capability_module(cap_name).nil?
+      !capability_module(cap_name.to_sym).nil?
     end
 
     # Executes the capability with the given name, optionally passing