Makes the execute method honor max_tries.

Vagrant::SSH.execute is hard coded for only 5 tries, this makes
  starting Debian machines very unreliable.  Debian will reset
  networking when you create a new hostonly NIC or change the hostname
  of the machine and occasionally the coming up part takes longer than
  what is provided by 5 tries.  Making this method honor the max_tries
  config option will make it possible to avoid this.

lib/vagrant/ssh.rb

@@ -98,7 +98,7 @@ module Vagrant
         exceptions = [Errno::ECONNREFUSED, Net::SSH::Disconnect]
 
         # Connect to SSH and gather the session
-        session = retryable(:tries => 5, :on => exceptions) do
+        session = retryable(:tries => env.config.ssh.max_tries, :on => exceptions) do
           connection = Net::SSH.start(env.config.ssh.host,
                          env.config.ssh.username,
                          opts.merge( :keys => [env.config.ssh.private_key_path],