This commit fixes issue hashicorp#6509, in which some subscription level
rights are required in order to create a key_vault. This occurred
due to the provider always looking for a soft deleted key vault
prior to checking whether the feature
KeyVault.RecoverSoftDeletedKeyVaults was set to true. Thus event
setting the value to false did not stop it searching for the soft
deletions.

The rights it was requiring were to perform action
'Microsoft.KeyVault/locations/deletedVaults/read' over scope
'/subscriptions/xxxxxx'

This commit fixes hashicorp#6509 by building on the previous commit to:
1) Remove the (now redundant) error message returned when there's
   a soft deleted vault of the same name but the user doesn't want
   to recover it. This is already covered by an error returned by
   Azure.
2) Fix the acceptance test to look for the error message Azure
   returns in this instance, rather than the text in the removed
   error.
3) Fixes the test to add a secret to the soft deleted vault, as
   Azure will otherwise silently recover the vault anyway (even if
   the mode is not 'recover') in this case.
4) Fixes the test so that it doesn't purge the vault on destroy.