oeqa/selftest: Fix imagefeature testcases not to interfere with testimage on AB
[YOCTO #8017] - selftest does not use pkill qemu
[YOCTO #7976] - tests do not touch .ssh/known_hosts
- don't hardcode qemu IP
[YOCTO #8027] - use qemu nographic
Extra: removed unnecessary assert for bitbake and runCmd status
Signed-off-by: Daniel Istrate <daniel.alexandrux.istrate@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>