Allow for configurable Jetty Configuration classes

Current Jetty Embeddable Container 7 works on all v7 and v8 releases,
but its usage of Jetty Configuration classes is hardcoded,
which conflicts with renamed classes in in v7.2+.

If this was configurable, user could change which Configuration classes should be used.

Perhaps providing both default would be good;
e.g. one for pre-7.2, one for 7.2+