Add maven stage yaml for 3.1.0 version

- Step 5 from https://wiki.onap.org/display/DW/CPS+Release+Process
- Change for CPS Core Only

Issue-ID: CPS-1277
Change-Id: I8819f6a73ab84f6e4794181920a903d36e107479
Signed-off-by: mpriyank <priyank.maheshwari@est.tech>
1 file changed