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