Make rabbit_vhost:add/2 idempotent

[michaelklishin]

Proposed Changes

This makes rabbit_vhost:add/2 idempotent. Returning an error when an existing vhost is [re-]created
is not productive and worsens CLI and automation tool experience.

Types of Changes

Technically it's a breaking change but we don't expect it to break a system that can handle
both success and error responses in a reasonable way.

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

Part of rabbitmq/rabbitmq-cli#260.

[#160792770]