Add Helm chart for the RabbitMQ connector

[welteki]

Description

Add a new Helm chart for the RabbitMQ connector.

Why is this needed?

• I have raised an issue to propose this change (required)

Allow users to deploy the OpenFaaS rabbitmq-connector with Helm.

Who is this for?

What company is this for? Are you listed in the ADOPTERS.md file?

How Has This Been Tested?

Deployed the connector in my local cluster using the chart.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

@welteki could you add a securityContext that is read from the chart?

The other connectors may not have this yet, but will do in time.