Merge branch 'make-custom-hook-dir-configurable' into 'master'

Make custom hook dir configurable

Add a new configuration option, custom_hook_dir. When this is set, we
will look for global custom hooks in: `<custom_hook_dir>/{pre-receive,update,post-receive}.d/*`

When this is not set, default to `<ROOT_PATH>/hooks`.

Relates to https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1754 and
gitlab-org/gitlab-ce!8040.

See merge request !113