[k6] Always pull latest cps and dmi stub images

Presently, k6 tests are using old cached DMI stub version in the
Jenkins job. This commit ensures latest images are pulled by
cleaning old images if a variable is set.

Issue-ID: CPS-2368
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: I66a749c7cdeb7daee06a00b06dff50ead6854f28
1 file changed