handing the proxy_redirect nginx directive

**Creating the possibility of handing the proxy_redirect nginx directive to fix an annoying jenkins reverse proxy broken warning**

Here is the jenkins help page when you get this reverse proxy broken issue https://wiki.jenkins-ci.org/display/JENKINS/Jenkins+says+my+reverse+proxy+setup+is+broken

If you're running nginx as a reverse proxy ( with SSL ) in front of plain-text HTTP jenkins now you can configure nginx's reverse_proxy directive to fix the above warning from jenkins
I've added a check for both `$proxy_redirect_tohost` and `$proxy_redirect_fromhost` inside vhost.pp, tested it in vagrant and updated this pull request.

When I omit any of these two parameters when using an nginx `$proxy` the I get this nifty little failure output:
```ruby
[jenkins] Running provisioner: shell...
[jenkins] Running: inline script
info: Loading facts in /vagrant/modules/jenkins/lib/facter/jenkins.rb
warning: Could not load fact file /vagrant/modules/jenkins/lib/facter/jenkins.rb: no such file to load -- puppet/jenkins/facts
<snip>
You must provide both $proxy_redirect_fromhost and $proxy_redirect_tohost at /vagrant/modules/nginx/manifests/resource/vhost.pp:287 on node jenkins.local
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!
```
Hope this is enough to get this pr merged !