Disable the stream_plugin if stream feature flag is not enabled

[Gsantomaggio]

Disable the stream_plugin if the feature flag is not enabled.
fixes #3557

Proposed Changes

fixes #3557

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• 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 an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

[Gsantomaggio]

How to test the issue:

 gmake virgin-test-tmpdir
RABBITMQ_FEATURE_FLAGS="implicit_default_bindings"  gmake run-broker  PLUGINS="rabbitmq_stream rabbitmq_stream_management" 

• Then try to access the management UI, the stream page should be not visible.
• Try to access using a stream client the 5552 port should be closed.

inside the logs you should see:

 Unable to start the stream plugin. The stream_queue feature flag is disabled. You need to enable it then disable and re-enable the rabbitmq_stream plugin.  See https://www.rabbitmq.com/feature-flags.html

same for the UI:

Unable to show the stream management plugin. The stream_queue feature flag is disabled.

following the instructions by enabling the feature flags:

sbin/rabbitmqctl enable_feature_flag all 

then:

sbin/rabbitmq-plugins disable rabbitmq_stream
sbin/rabbitmq-plugins enable rabbitmq_stream_management 

at this point, everything should work as expected.