azurerm_cdn_endpoint - defaulting the http_port and https_port

[tombuildsstuff]

As discovered by @changeworld in #288 - it appears http_port and https_port should be being defaulted to 80 and 443. From what I can tell this was previously done at the API - but are now mandatory parameters.

Also fixes the original documentation issue discovered in #288

Tests pass:

$ acctests azurerm TestAccAzureRMCdnEndpoint
=== RUN   TestAccAzureRMCdnEndpoint_importWithTags
--- PASS: TestAccAzureRMCdnEndpoint_importWithTags (185.24s)
=== RUN   TestAccAzureRMCdnEndpoint_basic
--- PASS: TestAccAzureRMCdnEndpoint_basic (166.18s)
=== RUN   TestAccAzureRMCdnEndpoint_disappears
--- PASS: TestAccAzureRMCdnEndpoint_disappears (179.67s)
=== RUN   TestAccAzureRMCdnEndpoint_updateHostHeader
--- PASS: TestAccAzureRMCdnEndpoint_updateHostHeader (196.97s)
=== RUN   TestAccAzureRMCdnEndpoint_withTags
--- PASS: TestAccAzureRMCdnEndpoint_withTags (193.53s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	921.610s

[grubernaut]

LGTM!

[ghost]

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