Rename backup-repository-config to backup-repository-configmap.
Rename repo-maintenance-job-config to repo-maintenance-job-configmap.
Rename node-agent-config to node-agent-configmap.
Add those three parameters to `velero install` CLI.
Modify the design and the site documents.
Signed-off-by: Xun Jiang <xun.jiang@broadcom.com>