cmd/management-console: only generate TLS cert/key if it does not exist
Created by: slimsag
Prior to this change this code had a bug in which we would always generate a
new TLS cert/key on startup, which prevented using your own unless you set
CUSTOM_TLS=true
(which is not desirable, setting that is only useful as a
"keep me sane that this thing won't overwrite my TLS cert/key" feature, not as
a requirement for using your own TLS cert/key pair).
Fixes #1683 (closed) Fixes #1507 (closed)