Adjustments to handle plugin updates using proper constraints

If a user provides the gem version using an explicit version or a
constraint, the update action should honor that constraint and not
simply replace it with an unbound constraint.

This also removes system plugin specifications from being matched
and preferred which prevents updates and can result in unexpected
downgrades when running the update.

lib/vagrant/bundler.rb

@@ -220,7 +220,7 @@ module Vagrant
       # Generate all required plugin deps
       plugin_deps = plugins.map do |name, info|
         if update[:gems] == true || (update[:gems].respond_to?(:include?) && update[:gems].include?(name))
-          gem_version = '> 0'
+          gem_version = plugins[name]["gem_version"].to_s.empty? ? "> 0" : plugins[name]["gem_version"]
           skips << name
         else
           gem_version = info['gem_version'].to_s.empty? ? '> 0' : info['gem_version']
@@ -275,12 +275,16 @@ module Vagrant
       # Create the request set for the new plugins
       request_set = Gem::RequestSet.new(*plugin_deps)
 
+      system_plugins = plugins.map do |plugin_name, plugin_info|
+        plugin_name if plugin_info["system"]
+      end.compact
+      installer_set.system_plugins = system_plugins
+
       installer_set = Gem::Resolver.compose_sets(
         installer_set,
-        generate_builtin_set,
+        generate_builtin_set(system_plugins),
         generate_plugin_set(skips)
       )
-
       @logger.debug("Generating solution set for installation.")
 
       # Generate the required solution set for new plugins
@@ -349,11 +353,13 @@ module Vagrant
     end
 
     # Generate the builtin resolver set
-    def generate_builtin_set
+    def generate_builtin_set(system_plugins=[])
       builtin_set = BuiltinSet.new
       @logger.debug("Generating new builtin set instance.")
       vagrant_internal_specs.each do |spec|
-        builtin_set.add_builtin_spec(spec)
+        if !system_plugins.include?(spec.name)
+          builtin_set.add_builtin_spec(spec)
+        end
       end
       builtin_set
     end
@@ -428,9 +434,11 @@ module Vagrant
     # the entire set used for performing full resolutions on install.
     class VagrantSet < Gem::Resolver::InstallerSet
       attr_accessor :prefer_sources
+      attr_accessor :system_plugins
 
       def initialize(domain, defined_sources={})
         @prefer_sources = defined_sources
+        @system_plugins = []
         super(domain)
       end
 
@@ -438,6 +446,11 @@ module Vagrant
       # for preferred sources
       def find_all(req)
         result = super
+        if system_plugins.include?(req.name)
+          result.delete_if do |spec|
+            spec.is_a?(Gem::Resolver::InstalledSpecification)
+          end
+        end
         subset = result.find_all do |idx_spec|
           preferred = false
           if prefer_sources[req.name]