Set MIDONET_SRC_DIR in midonetrc

Instead of setting MIDONET_SRC_DIR in one of the pre script file,
as it is useful throughout the midostack run, set it in the
midonetrc to make it available.  Note that this variable is not
something settable by the user.

Change-Id: I011c4a8efd09d2b412483888f9a41c27edb8bab5