MetastoreConf - String casting of default values breaks Hive

When using the getVar(Configuration conf, ConfVars var) method of the MetastoreConf class, Apache breaks when e.g. trying to retrieve the environment variable "USE_SSL" and it isn't set in the system. The method then tries to cast the default value, which is the boolean false for USE_SSL, to a String which can't work.

return val == null ? conf.get(var.hiveName, (String)var.defaultVal) : val;

Also in the getStringCollection(Configuration conf, ConfVars var) method it tries to cast any default values to a String.

Strangely, e.g. in the method get(Configuration conf, String key) the default value isn't casted but the .toString() method is called, which should be also done for the 2 methods I mentioned above.

If nobody has time for that fix, I could open a PR for that.