Add option to auto-create a Conjur data encryption key

[diverdane]

Is your feature request related to a problem? Please describe.

Currently, dataKey is a required chart value. Users are recommended to run the following
for manually creating a data encryption key:

```
DATA_KEY="$(docker run --rm cyberark/conjur data-key generate)"
```

and then this value should be configured either in a custom values.yaml file,
or with the following helm install ... command line flag:

--set dataKey="$DATA_KEY"

It should be possible to add a chart value to enable the auto-creation of the Conjur
data encryption key. The value might looks something like this:

database:
    autoGenerateDataKey: false

For implementing this feature, it may be possible to add the
auto-generation logic into the conjur-oss/templates/_helpers.tpl template file.

When this feature is implemented, a deprecation notice must be added to the
conjur-oss/templates/NOTES.txt if the auto-generation of the encryption key
is expected to become the default for the next major release.

Describe the solution you would like

• A chart value is available to have the Conjur data encryption key automatically generated.
• A deprecation warning shows up in NOTES.txt if auto-generation will become the default for next major release
  (unless user is enabling this feature, since deprecation warning is not necessary).

Describe alternatives you have considered

None.

Additional context