Add HTTP Proxy capability

[marinsalinas]

Closes #26

To add proxy_url just add to provider definition:

provider "nutanix" {
  username     = "xxxx"
  password     = "xxxx"
  endpoint     = "xxxx"
  insecure     = true
  port         = 9440
  wait_timeout = 10 //Optional
  proxy_url    =  "xxxx" //Optional
}