[scripts] Cleanup intermediate files

The installation scripts should not leave behind unused files created
during their run, but instead remove them on success:
- index.html, nginx-app.yaml generated by nginx testing;
- tiller-rba.yaml generated by tiller install;

Change-Id: Iebbfb13e0da6f9657dad65dc34cf62147210c487
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>