Incorrect XmlInstruction implementation of setting overwrite=true

The current XmlInstruction allows specifying overwrite=true, meaning that existing configuration will (or better: should) be replaced.

However the implementation incorrectly 'just' loads the configuration (through Session.importXML), without taking care (removal?) of the existing configuration first.

Which therefore either:

• fails if same-name-siblings are not allowed
• incorrectly 'succeeds' if same-name-siblings are allowed, then creating a second(ary) configuration sibling

Neither of the above is correct and this should be fixed or the support for override=true disabled, at least for now.

As none of the current Essentials plugins use/expect/need overide=true, and furthermore the proper handling needed requires further consideration (should it blindly delete the existing configuration, including possible user modifications?), for now we will disable the usage of override=true, and also clearly state this in the API javadoc.

When invoking XmlInstruction.setOverwrite(true) an UnsupportedOperationException will be thrown.