Changes for #482 to support setting/getting config props with spaces in the value

Made some changes which improve how the config/commands plugin iterates over the environment variable hash to make setting config variables containing spaces possible, for example after this change this works:

```
sudo dokku config:set myapp 'JAVA_OPTS="-Xmx384m -Xss512k -XX:+UseCompressedOops"' ARG2=VAL4
```