VSphere: Make API_PROVIDER format consistent with other drivers (DTACLOUD-399)
Vsphere: Return 400 in case that required parameter is missing

I discover this by creating an instance without name. The error
was reported as 500 (Backend error), but it should be 400 (Bad request).
VSphere: Make the 'name' parameter optional

In case 'name' is not sent, use the default "inst#{Time.now.to_i}"
name.
Vsphere: Fixed typo in create_instance method (DTACLOUD-401)

In case the driver method is called from CIMI, the opts[:image_id]
is empty. Lets use the 'image_id' here anyway.