[Enhancement] Support ImagePullPolicy

[yandongxiao]

Description

Support ImagePullPolicy.
Fixes: #518

Checklist

For operator, please complete the following checklist:

• run make generate to generate the code.
• run golangci-lint run to check the code style.
• run make test to run UT.
• run make manifests to update the yaml files of CRD.

For helm chart, please complete the following checklist:

• make sure you have updated the values.yaml
  file of starrocks chart.
• In scripts directory, run bash create-parent-chart-values.sh to update the values.yaml file of the parent
  chart( kube-starrocks chart).