add rg and name clash validation

[mjudeikis]

If resourceGroup exists and it is the same name as a cluster - the provider fails.

We should fail early.

[jim-minter]

The problem is valid but I'm not sure about the fix, so don't want to merge this yet. I actually think there are two options here:
1- end user decides the resource group name and we validate that it doesn't exist on create
2- we create a randomly named resource group.

For user experience I'm hoping we go for 1.

[mjudeikis]

End-user currently decides the resource group already and passes it in (CLI help and created it for the user). But if the user creates RG with name mycluster runs it with:
`az aro create -n mycluster -g mycluster, API will accept it and will fail, as
RP will try to create RG for the cluster. I think we would be better to do:

Allow customer to chose resource group name (and create it if it does not exist),
Create a cluster resource group with random post-fix

This would create these scenarios:

1. As a user, I have to create JUST VNET resource group: cluster-vnet
   RP creates me a resource group from -n flag or resourceName to host cluster object
   RP creates me a resource group clusterName-{random_5} and deploys the cluster.

2. As a user I have created VNET RG and cluster RG. cluster-rg, cluster-vnetRP creates cluster object insidecluster-rgresource group. RP creates cluster RG groupcluster-{random_5}`

I think we should add random postfix to the cluster ResourceGroup either way as it would solve these conflicts and would allow the user to use -n and -g with the same values

[jim-minter]

Create a cluster resource group with random post-fix

No - plan A is to allow the end user to specify the name of the cluster resource group. January.

Will merge a fix like this for now.