Added poller to check for consistency on role assignment creation

[jackofallops]

Enhancement to poll separate endpoint to improve eventual consistency checking.
Tests:

 --- PASS: TestAccAzureRMRoleAssignment/basic/emptyName (47.81s)
 --- PASS: TestAccAzureRMRoleAssignment/basic/roleName (46.57s)
 --- PASS: TestAccAzureRMRoleAssignment/basic/dataActions (46.86s)
 --- PASS: TestAccAzureRMRoleAssignment/basic/builtin (45.08s)
 --- PASS: TestAccAzureRMRoleAssignment/basic/custom (49.22s)
 --- FAIL: TestAccAzureRMRoleAssignment/basic/requiresImport (57.47s)
 --- FAIL: TestAccAzureRMRoleAssignment/assignment/group (53.51s)
 --- FAIL: TestAccAzureRMRoleAssignment/management/assign (15.46s)

Failures are expected, and not related to this change.

[tombuildsstuff]

otherwise LGTM 👍

[ghost]

This has been released in version 2.10.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.10.0"
}
# ... other configuration ...

[ghost]

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