Allow multiple deployments

[zambien]

fixes #28

The issue here occurred because we always took the latest deployment and returned it. Now we will select the actual deployment that is active. Note that this requires users to manage multiple proxy deployments if they want to do this. An example of this is:

apply 1:

resource "apigee_api_proxy" "tf1_helloworld" {
   name         = "tf1_helloworld"
   bundle       = "${path.module}/helloworld_proxy.zip"
   bundle_sha   = "${filebase64sha256("helloworld_proxy.zip")}"
}

resource "apigee_api_proxy_deployment" "foo_api_proxy_deployment" {
   proxy_name   = "${apigee_api_proxy.tf1_helloworld.name}"
   org          = "zambien-trial"
   env          = "test"
   revision     = "1"
}

apply 2:

resource "apigee_api_proxy" "tf1_helloworld" {
   name         = "tf1_helloworld"
   bundle       = "${path.module}/helloworld_proxy2.zip"
   bundle_sha   = "${filebase64sha256("helloworld_proxy2.zip")}"
}

resource "apigee_api_proxy_deployment" "foo_api_proxy_deployment" {
   proxy_name   = "${apigee_api_proxy.tf1_helloworld.name}"
   org          = "zambien-trial"
   env          = "test"
   revision     = "1"
}


resource "apigee_api_proxy_deployment" "foo_api_proxy_deployment2" {
   proxy_name   = "${apigee_api_proxy.tf1_helloworld.name}"
   org          = "zambien-trial"
   env          = "test"
   revision     = "latest" #or 2
   override     = true
}

I also made a fix for errors which would occur if using latest on a proxy deployment when the proxy was already deployed.

The output of terraform show after running both applies is:

# apigee_api_proxy.tf1_helloworld:
resource "apigee_api_proxy" "tf1_helloworld" {
    bundle       = "./helloworld_proxy2.zip"
    bundle_sha   = "bcRaaj0Vcp2biHPvQBiElVhopfY+h4ngZTQrhljIezE="
    id           = "adbc4519-f457-4601-9d95-697829cd60bb"
    name         = "tf1_helloworld"
    revision     = "2"
    revision_sha = "bcRaaj0Vcp2biHPvQBiElVhopfY+h4ngZTQrhljIezE="
}

# apigee_api_proxy_deployment.foo_api_proxy_deployment:
resource "apigee_api_proxy_deployment" "foo_api_proxy_deployment" {
    delay      = 0
    env        = "test"
    id         = "e721e174-843a-4f72-9c6c-3fea8918d574"
    org        = "zambien-trial"
    override   = false
    proxy_name = "tf1_helloworld"
    revision   = "1"
}

# apigee_api_proxy_deployment.foo_api_proxy_deployment2:
resource "apigee_api_proxy_deployment" "foo_api_proxy_deployment2" {
    delay      = 0
    env        = "test"
    id         = "2b01cba5-4366-4c3c-988c-e11d27edb85d"
    org        = "zambien-trial"
    override   = true
    proxy_name = "tf1_helloworld"
    revision   = "2"
}