Quick mod to kms client to attempt checking for keyring existence before attempting creation

[jbharter]

Where we currently don't want to grant the running service account full keyring creation permission on the project, we propose a change to first check for existence of the specified keyring. This means that the authenticating account for kms needs only permissions to modify the keyring it is given, and not the permission to create keyrings themselves.

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[jbharter]

@googlebot I signed it!

[chtyim]

Would you mind opening a JIRA at https://cdap.atlassian.net/browse/CDAP as well?

[aviachar]

Closing old PRs