Generate the gitlab's instance name from the api_baseurl by default

using the host of the given url.

This allows to create a lister task by simply specify the API base url
and prevent 'inconsistent by default' behavior, eg. with:

swh-scheduler task add swh-lister-gitlab-full \
    api_baseurl=https://0xacab.org/api/v4

the created task does not use 'gitlab' as instance name (but '0xacab.org'
here).

It's still possible to explicitely specify the instance name if needed.