Switch flag definitions to be on pflag instead of flag in `package go/cmd/vttablet`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/cmd/vttablet/*.go
go/cmd/vttablet/vttablet.go:    enforceTableACLConfig        = flag.Bool("enforce-tableacl-config", false, "if this flag is true, vttablet will fail to start if a valid tableacl config does not exist")
go/cmd/vttablet/vttablet.go:    tableACLConfig               = flag.String("table-acl-config", "", "path to table access checker config file; send SIGHUP to reload this file")
go/cmd/vttablet/vttablet.go:    tableACLConfigReloadInterval = flag.Duration("table-acl-config-reload-interval", 0, "Ticker to reload ACLs. Duration flag, format e.g.: 30s. Default: do not reload")
go/cmd/vttablet/vttablet.go:    tabletPath                   = flag.String("tablet-path", "", "tablet alias")
go/cmd/vttablet/vttablet.go:    tabletConfig                 = flag.String("tablet_config", "", "YAML file config for tablet")