[FLINK-943] Remove leading and ending double quotes from 'env.java.opts' config value

This is [FLINK-943](https://issues.apache.org/jira/browse/FLINK-943).

The parsing works correctly but the leading/ending double quotes need to be removed before the value is given to the `java` run command, e.g. `java -Xmx1g "-D ..."` does not work.

I've tested: 1. with double quotes, 2. without double quotes, and 3. not set.