AzureEnvironment Support for Managed Clusters

[luthermonson]

Goals

1. Add a new string to managed cluster spec identical to this in azureclusterspec
2. Pass through the AzureEnvironment for the managed cluster to the scope creation for example both here and here. This is identical behavior to the self-managed cluster.

User Story
As a user I would like to create managed AKS clusters in Germany/China/USGovernment cloud environments.

Detailed Description
It looks like support for AzureEnvironment was done in self-managed clusters and hardcoded to the empty string defaults in the AKS code paths. The code changes would allow access to all the environments offered by Azure.

Contract changes [optional]

[Describe contract changes between Cluster API Azure Provider controllers, if applicable.]

Data model changes [optional]
Will probably add an AzureEnvironment to azuremanagedcluster identical to the one in azurecluster

/kind proposal

[luthermonson]

I am already working on a PR for this and the issue is to get attention of maintainers to figure out if I'm misguided in the change. It appears straight forward and I'm testing the idea in a fork we maintain of CAPZ and have a govcloud account to test in. I'll confirm everything is functional and submit a PR when I feel confident.