Improve --json behaviour with interactive prompts

**What steps did you take:**

Create an ``example.yaml`` file containing:

```
apiVersion: v1
kind: ConfigMap
metadata:
name: xxx
data:
xyz: abc
```

Run:

```
kapp deploy -a website -f example.yaml --json
```

**What happened:**

It appeared to crash and generate a Golang stack trace.

```
{
"Tables": [
{
"Content": "",
"Header": {
"age": "Age",
"conditions": "Conds.",
"kind": "Kind",
"name": "Name",
"namespace": "Namespace",
"op": "Op",
"op_strategy": "Op st.",
"reconcile_info": "Ri",
"reconcile_state": "Rs",
"wait_to": "Wait to"
},
"Rows": [
{
"age": "",
"conditions": "",
"kind": "ConfigMap",
"name": "xxx",
"namespace": "lab-carvel-ytt-01-w01-s002",
"op": "create",
"op_strategy": "",
"reconcile_info": "",
"reconcile_state": "",
"wait_to": "reconcile"
}
],
"Notes": [
"Op: 1 create, 0 delete, 0 update, 0 noop",
"Wait to: 1 reconcile, 0 delete, 0 noop"
]
}
],
"Blocks": null,
"Lines": [
"Target cluster 'https://10.96.0.1:443'"
]
}panic: Cannot ask for confirmation in JSON UI

goroutine 1 [running]:
github.com/cppforlife/go-cli-ui/ui.(*JSONUI).AskForConfirmation(0xc0003daa00, 0x1535358, 0xc000cad1d0)
github.com/cppforlife/go-cli-ui@v0.0.0-20200716203538-1e47f820817f/ui/json_ui.go:114 +0x39
github.com/cppforlife/go-cli-ui/ui.(*ConfUI).AskForConfirmation(0xc0003ebd80, 0xc0001be140, 0xc0000791a0)
github.com/cppforlife/go-cli-ui@v0.0.0-20200716203538-1e47f820817f/ui/conf_ui.go:105 +0x33
github.com/k14s/kapp/pkg/kapp/cmd/app.(*DeployOptions).Run(0xc00012cc80, 0x0, 0x0)
github.com/k14s/kapp/pkg/kapp/cmd/app/deploy.go:171 +0xc8f
github.com/k14s/kapp/pkg/kapp/cmd/app.NewDeployCmd.func1(0xc00013f340, 0xc00007df90, 0x0, 0x5, 0xc000374580, 0x1527ef0)
github.com/k14s/kapp/pkg/kapp/cmd/app/deploy.go:54 +0x2a
github.com/k14s/kapp/pkg/kapp/cmd.NewKappCmd.func1.1(0xc00013f340, 0xc00007df90, 0x0, 0x5, 0x0, 0x0)
github.com/k14s/kapp/pkg/kapp/cmd/kapp.go:126 +0xb8
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc00013f340, 0xc00007df90, 0x0, 0x5, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc00013f340, 0xc00007df90, 0x0, 0x5, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc00013f340, 0xc00007df90, 0x0, 0x5, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/spf13/cobra.(*Command).execute(0xc00013f340, 0xc00007df40, 0x5, 0x5, 0xc00013f340, 0xc00007df40)
github.com/spf13/cobra@v1.1.1/command.go:850 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0xc00013e580, 0xc00013e580, 0xc0003ebd80, 0x7f73f8ce0108)
github.com/spf13/cobra@v1.1.1/command.go:958 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v1.1.1/command.go:895
main.nonExitingMain(0x0, 0x0)
github.com/k14s/kapp/cmd/kapp/kapp.go:42 +0x167
main.main()
github.com/k14s/kapp/cmd/kapp/kapp.go:21 +0x26
```

**What did you expect:**

Am not sure. I was trying to work out what the ``--json`` option actually did when it occurred, so not sure if this is expected and normal, but it sort of looks wrong.

**Anything else you would like to add:**

Nope.

**Environment:**

```
$ kapp --version
kapp version 0.37.0
```

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