Server block LWRP and nxensite/nxdissite script

Created a LWRP for creating, deleting, enabling and disabling nginx server blocks. And added site enable / disable script written by @perusio
For blocks like this, I prefer the following style:

```ruby
describe file("/usr/sbin/nxensite") do
  it { should be_a_file }
  it { should be_owned_by "root" }
  # ...
end
```