Set $BASE before $GATE_SCRIPT_DIR.

* devstack-vm-gate-wrap.sh: $BASE was used to construct $GATE_SCRIPT_DIR
but $BASE was set after $GATE_SCRIPT_DIR. Move the order of assignments
to fix this.

Change-Id: Ic6ee8f7f0a1db0c191e37fd3eec1d970752c8db0