Conditionals: string is allowed as conditional statement

Current grammar allows using string as conditionals statements like

```
if "i'm a fool" {
noop {
add_tag => "fool"
}
}
```

But as reported in LOGSTASH-1434, this could be confusing to non-programmer/distracted users that would write

```
if [field] == "string1" or "string2" {
....
}
```

and do not understand why the condition is always matched...