GEOMESA-151 allowing schema to not be set in datastore config for already created features

Since geoserver doesn't even have the ability right now to configure a custom schema.
Also, since validated above is a mutable.map, you could refactor 
```
    val ok = validated.get(featureName).getOrElse({
      val errors = checkMetadata(featureName)
      validated.put(featureName, errors)
      errors
    })
```
as
```
    val ok = validated.getOrElseUpdate(featureName, checkMetadata(featureName))
```
Assuming that checkMetadata is changed to return the entire error string.