Auth updates

[hattan]

This change affects the auth mechanism and specifically introduces the following:

• If Az.Powershell is connected, skip all auth and use existing connection. This also removes the need to set environment variables if the connection is existing.
• If environment variables are set, check that subscription id, tenant ip and application id match the current context, if not, throw an error.
• Remove the need to specify a subscription id for managed identities. It can still be passed it as optional if there is a specific subscription that needs to be used.

[PabloZaiden]

All good, just one comment