Added servercert param for SHA1 fingerprint of trusted server's SSL cert. 

I appreciate you publishing this puppet module (it saved me having to write one from scratch) so I figure I might as well submit a pull request for my changes in the case you wanted them. Either way, thanks for the module!

__Module Change Details:__ I added an optional param `openconnect::servercert` which allows you to use the openconnect shell command's `--servercert` option. This allows you to accept an openconnect server's SSL certificate ONLY if it's fingerprint matches the SHA1 hash provided in the `openconnect::servercert` param. Below is the description of the `--servercert` option from the openconnect man page:   
```plain
       --servercert=SHA1
              Accept server's SSL certificate only if its fingerprint matches SHA1.
```