Move repo ENV_VARS to midonetrc

Move the ENV_VARS that are being overriden by input parameters to
midonetrc file. Give them the option to be overriden too by 'export'
calls before to run the ./midonet_stack.sh script using the
VAR=${VAR:-default} syntaxis.

Change-Id: I922c149f0f66e583d27964b181fb764c61089ecf