Added vhost, user and exchange datasources

[Skeen]

Fixes #35

This PR introduces data sources for rabbitmq_user, rabbitmq_vhost and rabbitmq_exchange that can be used as such:

data "rabbitmq_user" "test" {
  name = "mctest"
}

data "rabbitmq_vhost" "vhost" {
  name = "my_vhost"
}

data "rabbitmq_exchange" "test" {
  name  = "test"
  vhost = data.rabbitmq_vhost.vhost.name
}

Corresponding with these resources:

resource "rabbitmq_user" "test" {
  name     = "mctest"
  password = "foobar"
  tags     = ["administrator", "management"]
}

resource "rabbitmq_vhost" "vhost" {
  name = "my_vhost"
}

resource "rabbitmq_exchange" "test" {
  name  = "test"
  vhost = rabbitmq_vhost.vhost.name

  settings {
    type        = "fanout"
    durable     = false
    auto_delete = true
  }
}

[taybin]

Can we get this merged? Is there a problem with the PR?

[cyrilgdn]

Thanks for your work on that, and sorry for the delay.