Improve TLS-related rabbitmq.conf settings of amqp_client (backport #15772) (backport #15773)

[mergify]

This is #15767 by @lukebakken with some tweaks from me:

1. A few places quietly returned undefined instead of using cuttlefish:unset/0
2. The SNI key used cuttlefish:unset/0 where it should have used disable when the value is set to none in rabbitmq.conf (the current behavior ends up deriving the SNI from the local hostname (a dubious behavior in the Erlang/OTP's ssl or inet apps if you ask me)
3. Added a bunch of config_schema_SUITE test cases for less commonly used TLS-related keys

References

1. cuttlefish tls schema for amqp_client #11415
2. Revert "cuttlefish tls schema for amqp_client" #11531
3. Re-submit cuttlefish tls schema for amqp_client #15767

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   This is an automatic backport of pull request Improve TLS-related rabbitmq.conf settings of amqp_client #15772 done by Mergify.

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   This is an automatic backport of pull request Improve TLS-related rabbitmq.conf settings of amqp_client (backport #15772) #15773 done by Mergify.