Make JAVA_HOME value available for Robot

Make JAVA_HOME value available for Robot

Previously, JAVA_HOME was only set temporarily to start ODL.
But if a Robot suite wishes to re-start ODL, it needs to know
the desired JAVA_HOME huitable for given stream.

This moves JAVA_HOME construction to *-get-bundle-vars
and includes -v JAVA_HOME:${JAVA_HOME} option to pybot.

TODO: Make also non-robot places of JAVA_HOME construction
to refer to the prepared value.

Change-Id: I5b2d805ee0171e62b5fabef269c2c75b557409d2
Signed-off-by: Vratko Polak <vrpolak@cisco.com>