Merge branch '128-fix-stunnel' into 'master'

Use 'exec' for clean stunnel shutdown

Closes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/128

Missing 'exec' caused Foreman not to terminate stunnel when shutting down.

See merge request !157

support/workhorse-stunnel

 #!/bin/sh
 main() {
-  stunnel -fd 3 3<<EOF
+  exec stunnel -fd 3 3<<EOF
 foreground = yes
 [workhorse-https]