Support ssl_cert_dir config setting

This MR introduces the ssl_cert_dir configuration setting that allows a directory to be specified that contains custom SSL certs, e.g

Setting ssl_cert_dir in config.yml to:

ssl_cert_dir: /tmp/certs

Will result in the 'authorised key' line containing the SSL_CERT_DIR env var as part of the command="..." section:

command="SSL_CERT_DIR=/tmp/certs <gdk-root>/gitlab-shell/bin/gitlab-shell key-21",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty ssh-rsa <ssh_public_key>

This MR works alongside:

• omnibus-gitlab!4379 (merged)
• gitlab!35839 (closed)

Related: #459 (closed)