Need to handle geoserver's envrionment variable in better way.

[ywkim312]

Currently, geoserver's env variables are being used separately in the chart. Make those in the one place so make it to easier to modify or check.
For example, it is currently like

cors:
  enabled: true

extension:
  install: false
  stableExtension: ""

demoData:
  skip: true

envs:
  extraJavaOpts: "-Xms512m -Xmx2g"

but it could be better if it goes like

env
  cors:
    enabled: true
  extension:
    install: false
    stableExtension: ""
  demoData:
    skip: true
  extraJavaOpts: "-Xms512m -Xmx2g"

and also in the deployment yaml file, it might be better to use something like

- name: EXTRA_JAVA_OPTS
   value: {{ .Values.envs.extraJavaOpts | quote }}
{{ with .Values.envs }}
            - name: .key
               value: .value
{{ end }}