Feat/presets

[monrax]

This adds support for the following top-level values:

• version: set both Graylog and DataNode versions in one go.
• provider: render provider-specific resources/fields.
• size: set number of replicas according to a size preset defined in Support Cluster Size Presets #23 (CPU and memory are NOT included in this PR -- they will be addressed in a future PR).

Notes for Reviewers

• The commit history must be preserved - please use the rebase-merge or standard merge option instead of squash-merge
• Sync up with the author before merging

How to test?

Version

helm install graylog ./graylog -n graylog --set version='6.3'

kubectl describe sts graylog -n graylog
kubectl describe sts graylog-datanode -n graylog

# both graylog and datanode containers should have 6.3 tag

Providers

Nothing to test for now.

Replicas

helm install graylog ./graylog -n graylog --set size=poc

kubectl get pods -n graylog

# only one graylog pod and one datanode pod should be scheduled

[monrax]

@williamtrelawny this has been rebased on top of the current update/docs branch. Please take a look at PR #24 before reviewing this one!

[monrax]

Turning this into a draft PR until #24 is merged.

[alix-graylog]

Ship it!