`config import` does not accept integer and fails with unspecific error.

[bartTC]

If your JSON import file contains integers, the import fails with a rather unspecific Go error.

{
  "SITE_ID": 1
}

$ apppack config import --overwrite -a appname ~/Desktop/test.json 
✖ json: cannot unmarshal number into Go value of type string

I also think this is a regression, since the setup routines I use here haven't changed. Since ENV variable values are strings anyway, I think this requirement is OK, but should be more clear and ideally tell me, which value is invalid, and what to do.

This JSON will work:

{
  "SITE_ID": "1"
}