Allow protocol override when overriding the default host

This allows overriding the default protocol of the hostname if a fully qualified url is provided when using the GITLAB_HOST environment variable. For example, if the GITLAB_HOST=http://my.glinstance.org, then the host will be set to my.glinstance.org and the protocol become http, unlike before when the protocol was ignored and only the hostname was considered.

This is a follow-up for #678 and #667