Add the ability to do basic filtering with azurerm_subscriptions

[tiwood]

This adds two new optional parameters to the data source azurerm_subscriptions:

• display_name_prefix
• display_name_contains

Example:

data "azurerm_subscriptions" "current" {
  display_name_prefix = "Sub_"
  display_name_contains = "_123"
}

output "available_subscriptions" {
  value = "${data.azurerm_subscriptions.current.subscriptions}"
}

This gives:

{
        display_name = Sub_FooBar_123,
        location_placement_id = Public_2014-09-01,
        quota_id = AAD_2015-09-01,
        spending_limit = On,
        state = Enabled,
        subscription_id = foo
}

[tombuildsstuff]

hey @tiwood

Thanks for this PR :)

I've taken a look through and left some comments inline but this mostly LGTM - if we can fix up the comments (and the tests pass) this otherwise LGTM 👍

Thanks!

[tombuildsstuff]

rather than iterating over the results multiple times, I'd suggest it'd be easiest to do this check here - what do you think?

[tiwood]

I moved the check to the bottom and simplified it a bit (check against the slice val). This saved a few lines of code, but I'm not sure if this is what you meant. 🤓️

[tombuildsstuff]

rather than iterating over the results multiple times, I'd suggest it'd be easiest to do this check here - what do you think?

[tiwood]

see above.

[tombuildsstuff]

Suggested change

	* `display_name_prefix` - (Optional) This argument can be used to only return subscriptions that start with this given string.
	* `display_name_prefix` - (Optional) A case-insensitive prefix which can be used to filter on the `display_name` field

[tiwood]

thanks @tombuildsstuff, I made the requested changes.

[tombuildsstuff]

LGTM - thanks for this @tiwood

(given this test is based on the Subscription name, we're unable to add tests for these at this time since we're not able to dynamically provision subscriptions at the moment - we'll circle back to this when we can)

[tombuildsstuff]

Tests pass:

[ghost]

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