One more place where a map definition must be converted to proplist before validation

[michaelklishin]

Proposed Changes

This covers one more place where a map definition must be converted to proplist before validation.
Originally reported in rabbitmq/rabbitmq-management#565.

Types of Changes

• Bug fix (non-breaking change which fixes issue Operator policy validation fails for no good reason during definition import rabbitmq-management#565)
• 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

Fixes rabbitmq/rabbitmq-management#565.

References #1493, rabbitmq/rabbitmq-federation#70,
rabbitmq/rabbitmq-shovel#38, rabbitmq/rabbitmq-federation#73.

[#157045132]