[gitlab] Autodetect gitlab enterprise-url arg

[Alch-Emi]

The enterprise-url parameter of the gitlab backend is currently left to be specified by the user in the config file. This commit removes this burden from the user by automatically detecting the enterprise URL from the url passed to get_perceval_params_from_url()`.

This aims to make it easier for users to work with enterprise GitLab instances by removing the need to use the enterprise-url at all.

As far as I know, this does not introduce any backwards-compatibility issues.

[coveralls]

Pull Request Test Coverage Report for Build 1121968729

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 82.237%

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Files with Coverage Reduction	New Missed Lines	%
/home/runner/work/grimoirelab-elk/grimoirelab-elk/grimoire_elk/raw/gitlab.py	2	84.62%
<!--	Total:	2		-->

Totals
Change from base Build 1052244504:	-0.02%
Covered Lines:	8667
Relevant Lines:	10539

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💛 - Coveralls