[Feature/Operator] Manage configmap and secret for override

[erdrix]

Q	A
Bug fix?	yes
New feature?	yes
API breaks?	yes
Deprecations?	yes
Related tickets	fixes #75
License	Apache 2.0

What's in this PR?

It contains an improvement in configuration management :

• The configuration files are no longer stored in a configmap but in a secret (it allows to secure sensitive information that are stored in the configuration)
• Added possibility for the .properties files to override the generated configurations providing a configmap or a secret in addition to the OverrideConfigs field into the NifiCluster resource.
• Added possibility to replace default logback.xml and bootstrap-notification-servces.xml files with a secret or configmap content (secret is prior to the configmap if both are specified)

It also contains a fix that correctly catch configuration changes to trigger the rolling update configuration.

Why?

These new features allow to deal with sensitive information available in configuration, and give more flexibility in configuration management (as proposed in #75)

Checklist

• Implementation tested
• Error handling code meets the guideline
• Logging code meets the guideline
• User guide and development docs updated (if needed)
• Append changelog with changes

[teplydat]

@erdrix thank you very much. Looks very well, I already tested it and basically it works.

Two things that I faced with it:

• I got something like "logbackConfig does not exists in readOnlyConfig" when I tried to apply it via kubectl. I used:

spec:
  readOnlyConfig:
    logbackConfig:
...

It seems the crd is old in the helmchart of 0.6.0.
After recompiling + applying the crd, it just worked like a charm.

• I just wanted to point the nifikop to namespace "default". But nifikop wants to list namespaces clusterwide (not covered by the helm chart - but I am also not sure why nifikop needs cluster permissions to list namespaces)...