Allow string value for grails.redis.port in properties file

Hey! Hopefully you're not sick of all my pull requests yet. So my application overwrites certain Config.groovy properties in an external properties file. There may be a need for to overwrite the ```grails.redis.host``` and ```grails.redis.port``` in that file. Since it is a ```.properties``` file instead of ```.groovy``` all of the properties are read in as strings. The ```port``` is required to be an ```Integer``` so I added a check that if the property is a ```String``` it converts it to an ```Integer```.

Let me know what you think. Thanks