Add ability to login an extension user with a certificate

[sbacheld]

Today, the only way to connect to the vCenter Server using the rbvmomi gem is using the Login function. VMware also provides the option to login an extension user using a certificate (LoginExtensionByCertificate). If you're not familiar, an extension user is added to the vCenter Server using the vSphere client by registering a plugin in the "Manage Plug-ins" dialog.

You can see an example plugin, public certificate, and private key combo here: https://gist.github.com/3030573

With the changes I've made, you can login to the VC with the following command:

vim = RbVmomi::VIM::connect_extension_by_certificate proxyHost: 'vcenter server ip', extension_key: 'My_Extension_Key', insecure: true, cert: "server.crt", key: "server.key"

I verified that I was able to retrieve information from the vCenter Server after logging in

Let me know if you need any more information

Thanks,
Sean