Add `gitlab-ctl reload` or `gitlab-ctl graceful-restart`

The following discussion from !3641 (merged) should be addressed:

• @jarv started a discussion: (+17 comments)

  Note that gitlab-ctl hup unicorn does not actually send a HUP, but rather translates it into a USR1 https://gitlab.com/gitlab-org/omnibus-gitlab/blob/30cd830b631ce692dd1fbe55f216adf16533a92a/files/gitlab-scripts/gitlab-unicorn-wrapper#L123-126

  We may want to clarify that here.

Proposal

Make the following changes:

• Change behavior of gitlab-ctl restart to be a real restart,
• Add gitlab-ctl reload to be a something that reasembles graceful reload where possible,
• Update our zero-downtime documentation to refer to usage of gitlab-ctl reload everywhere, and make service change due to gitlab-ctl reconfigure to use gitlab-ctl reload instead of restart