Require client to be set and provide stub on initial startup

When running in server mode, make the Environment validate a client
option is provided on instantiation. If no client option is provided,
raise an exception since it is required in server mode.

Since an initial Environment is required during start, include a
stub value for the client so the initial Environment can be instantiated
successfully and the GRPC service can be started.

bin/vagrant

@@ -197,6 +197,12 @@ begin
   # Check if we are running the server
   if sub_cmd == "serve"
     Vagrant.enable_server_mode!
+    # NOTE: We stub the client option to allow the environment to
+    #       be initialized for startup since it will raise an
+    #       exception if it is unset. We don't care about the
+    #       initially created environment, we just need it long
+    #       enough for our serve command to get executed.
+    opts[:client] = :stub
   end
 
   # Create the environment, which is the cwd of wherever the

lib/vagrant/environment/remote.rb

@@ -11,6 +11,10 @@ module Vagrant
       def initialize(opts={})
         super
         @client = opts[:client]
+        if @client.nil?
+          raise ArgumentError,
+            "Remote client is required for `#{self.class.name}'"
+        end
       end
 
       # Gets a target (machine) by name
@@ -18,7 +22,7 @@ module Vagrant
       # @param [String] machine name
       # return [VagrantPlugins::CommandServe::Client::Machine]
       def get_target(name)
-        return @client.target(name)
+        client.target(name)
       end
     end
   end