Make SingleFeatureTest max heap size configurable

Make SingleFeatureTest max heap size configurable

Currently, CSIT jobs of -only- type use 2GB of heap and CSIT
of -all- type use 3GB. But we are seeing occasional failures
in distribution, especially odl-integration-all feature can take
more heap than usual.

Address the TODO item by allowing maximum heap size to be specified
as a system property -- thus can be overridden by downstreams in
their pom.xml.

Change-Id: I84adda01f3bd20180c6d81d3da807d38b5af6fa1
Signed-off-by: Vratko Polak <vrpolak@cisco.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>