Add standard ca certificate paths

When using the uri module in a base job it cannot validate ssl certs
unless you add the ca certificate paths to (un)trusted-ro-paths. This
seems a common use case so it makes sense to mount them into the bwrap
context by default if they are existing.

Change-Id: I2277374cdb8455dd9e39222ef0ecbab4c8ac786e

zuul/driver/bubblewrap/__init__.py

@@ -213,6 +213,7 @@ class BubblewrapDriver(Driver, WrapperInterface):
                      '/etc/nsswitch.conf',
                      '/etc/lsb-release.d',
                      '/etc/alternatives',
+                     '/etc/ssl/certs',
                      ]:
             if os.path.exists(path):
                 bwrap_command.extend(['--ro-bind', path, path])