Datasources & standard documentation website setup

[ringods]

@amfranz can you merge this last PR? As there is no evolution in handing over this code base to an official XenServer or XenAPI "organisation", I'm willing to have it transferred to my account. But for the transfer to succeed, I need to remove my fork first. This PR contains my latest changes. When this is merged, we can start the process of the Github repo migration.

Content:

• switch from glide to govendor dependency management to be in line with all the other TF provider implementations.
• Initial code for the xenserver_pifs datasource
• File system structure for the provider website documentation.

[ringods]

@amfranz last PR before a possible transfer of your repos. The earlier you can do this, the sooner you are freed of this project and we can go forward. 😉

[ringods]

Ignore my last comment. I forgot you made me a contributor. 😃