Send SIGTERM repeatedly to terminate trillian processes

Repeated kill is a workaround that can most likely be reverted once https://github.com/google/trillian/pull/2805 makes into into a trillian release.

Uses SIGTERM rather than SIGINT, because (i) SIGTERM (the default kill signal) seems more appropriate for scripted usage, while SIGINT is for interactive usage, e.g., pressing Control-C in the terminal, and (ii) SIGINT didn't work in my local tests.

Based on patch by Grégoire Détrez.