hs-test: use equal ubuntu versions in test images

Official nginx image is based on Debian with older libc version,
that causes a runtime fail when VPP libraries are compiled in Ubuntu
which has newer libc.
Using equal version of Ubuntu in VPP image and in nginx image
ensures that running nginx won't fail due to different libc versions.

Type: test
Signed-off-by: Maros Ondrejicka <maros.ondrejicka@pantheon.tech>
Change-Id: I48f3b23be30a9d9d9144351437ce163d64a4bb6b