Added default value for GITLAB_INSTANCE_URI

[AgusRdz]

Description

This commit solve the issue when a user try to sync your GitLab repositories,

Motivation and context

Currently GitLab is thorowing an error while a user try sync your repositories, this is caused because the env helper isn't working properly on app/Classes/GitLab, also the function tplRepository into this file is trying to access to $repo->public but this property not exists and now is using $repo->visibility instead, visibility property is an string not a boolean as some users suggests in others PR.

fixes #329

How has this been tested?

Performed several tests manually with a fresh repo installation and via postman to debug the API response.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.