Update DMI demo stub version and container image configuration

Update DMI demo stub version and container image configuration

- Updated the DMI_DEMO_STUB_VERSION to `1.8.0-SNAPSHOT` in `.env` and `test.properties`.
- Modified `ncmp-dmi-plugin-demo-and-csit-stub` Docker Compose service to reference:
    - Custom container name using `${NCMP_DMI_PLUGIN_DEMO_AND_CSIT_STUB_CONTAINER_NAME}`.
    - Image from `${DOCKER_REPO}/onap/dmi-stub:${DMI_DEMO_STUB_VERSION}`.
- Ensures consistent version control and improved flexibility in image source and container naming.

Issue-ID: CPS-2837
Change-Id: Ic2951dee646b900294e019a4fee021bf5d875b01
Signed-off-by: sourabh_sourabh <sourabh.sourabh@est.tech>