Allow Existing KeyVault to process withput breaking

[msalemcode]

If user passed KeyVault name or Accelerator deployed over existing resouces, Script will exist due to existing KeyVault and direct user to purge KeyVault.

Solution:
1- If user provided KeyVault name, Deployment script will by pass the validation. It is user responsibility to provide existing valid KeyVault name
2- If Deployment used auto generated keyvault name then there is two different actions:

• If KeyVault exists then it will pass and proceed with deployment.
• If KeyVault does not exist then it will check if the KeyVault was deleted before or not.

[neelavarshad]

@msalemcode The validation is right, although the script could still fail, if the user provides the name of a keyvault that already exists.
At the time of deploying azure resource (after preparing publish files), we are calling az create again on the same keyvault name, there should be a check to ensure we are not recreating keyvault if it already exists.