[incubator/kafka] SSL support - Disable env from secret when secret is mounted

[kaarolch]

What this PR does / why we need it:
This PR allow to mount binary files like jkms during kafka deployment. This PR allow you to enable SSL inside kafka with jks. The previous version of charts by default try to read environment variables and also mount files from secret. When the secrets include some binary data container throw error during start:

writing syncT run type caused \"write parent: broken pipe\"

Because we didn't found any use case when someone needs to mount files from secret and in the same time load it as env. variable, the proposed changes do not add env. variable when mountPath was specified.
After this PR we can use secrets with jks or pem files and configurationOverrides

 configurationOverrides:
    ....
    "ssl.keystore.filename": server.jks
    "ssl.keystore.credentials": server.keystore.txt
    "ssl.truststore.filename": ca_test.truststore.jks
    "ssl.truststore.credentials": ca_test.truststore.txt

to enable SSL support.

**Which issue this PR fixes: #3951

Special notes for your reviewer:
If there is a special case why secrests need to be mounted and loaded as env variable we can propose refactoring of secrets section in helm:

## Useful if using any custom authorizer
## Pass in some secrets to use (if required)
# secrets:
# - name: myKafkaSecret
#   keys:
#     - name: username
#       type: file
#     - name: password
#       type: value
#   # mountPath: /opt/kafka/secret

When type is defined as file we can skip add env variable. Unfortunately this change broke compatibility with previous version.

[kaarolch]

/assign @benjigoldberg

[benjigoldberg]

/ok-to-test

[benjigoldberg]

@kaarolch this is a nice catch and fix. Thank you for taking the time to submit a patch for this. I agree with you, I can't think of any obvious reason why you would need both and they conflict if used together. LGTM.

[benjigoldberg]

/lgtm

[k8s-ci-robot]

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[encryptblockr]

@kaarolch
you mind sharing your values.yaml and possibly statefulset.yaml files you used to setup SSL for kafka?
even if it requires replacing sensitive info with dummy values for security/privacy

[amitsadafule]

In case anybody wants to setup SSL auth, I have documented the steps on below link
https://medium.com/inspiredbrilliance/helm-incubator-kafka-setup-with-ssl-auth-7cf6fbe333d4