Azure Active Directory Graph API, now deprecated, does not provide
support for atomically creating/removing passwords on an application. As
a result, there is a race conditions that can occur when creds are being
created for roles configured with an existing service principal that
is configured on multiple mounts or across multiple Vault clusters.

Unfortunately,
[`Azure/azure-sdk-for-go`](https://github.com/Azure/azure-sdk-for-go)
does not yet offer a MS Graph API client, therefore, this PR utilizes
[`Azure/go-autorest`](https://github.com/Azure/go-autorest) to construct
a client the same as
[`Azure/azure-sdk-for-go`](https://github.com/Azure/azure-sdk-for-go).

This changeset preserves using the AAD Graph API by default but provides
a mount configuration option for toggling to the new MS Graph API. This
is because the two APIs require different API permissions. This allows
users to upgrade to the new plugin version and then switch to the new
API.

Additionally, although using the MS Graph API is a net benefit, it
itself has reliability issues when handling multiple requests in
parallel. More details can be found in
https://github.com/mdgreenfield/microsoft-graph-api-reliability and I am
working with Microsoft to try to get some of these reliability issues
resolved.

Fixes #58