java: isolated contexts should not inherit system properties

Context started from another context should not inherit system
properties because they may interfere with its state badly. For
example inheriting of logging properties may result in
ClassNotFoundException:s in the child because the classes present in
parent context are not found in the child context.

The initial system properties set by JRE should be available though.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>