executor: run trusted playbook in a bubblewrap

This change renames untrusted_wrapper to execution_wrapper and uses
bubblewrap for both trusted and untrusted playbooks by default.

This change adds new options to the zuul.conf executor section to let
operators define what directories to mount ro or rw for both context:
* trusted_ro_dirs/trusted_rw_dirs, and
* untrusted_ro_dirs/untrusted_rw_dirs

Change-Id: I9a8a74a338a8a837913db5e2effeef1bd949a49c
Story: 2001070
Task: 4687
7 files changed