feature: support ConfigMap and Secret by changing env from map to EnvVar

[ptyin]

The older method to configure SeataServer is to use env property like following,

apiVersion: operator.seata.apache.org/v1alpha1
kind: SeataServer
# ...
spec:
  # ...
  env:
    console.user.username: seata
    console.user.password: seata

However, the env property is just a map, it does not support valueFrom like Kubernetes EnvVar. We would like to refactor the current env implementation from map type to EnvVar so to support ConfigMap and Secret. The current form is something like following,

apiVersion: operator.seata.apache.org/v1alpha1
kind: SeataServer
# ...
spec:
  # ...
  env:
  - name: console.user.username
    value: seata
  - name: console.user.password
    valueFrom:
       secretKeyRef:
         name: seatapwd
         key: console

Fix #15

[slievrly]

LGTM