Allow for Jenkins project/folder level GitLab Token API credentials to be used for GitLab connection

## Feature Request
In our enterprise we have an Enterprise GitLab and an Enterprise Jenkins set up such that groups have their own Jenkins folders and don't have access to the Global Jenkins configuration to add their GitLab API Token credentials for their GitLab groups projects at the global level. It appears that the gitlab-plugin pulls the GitLab connection info from the Manage Jenkins->Configure admin level which only the Enterprise Jenkins admin has access to. If the GitLab API Token was pulled from the project/folder/group level vs. the admin level then non-admins would be able to add their own GitLab API Token without requesting the Jenkins admin to add it at the global level.

### Context
- **Gitlab plugin version**: 1.4.2
- **Gitlab version**: Community Edition 9.5.1
- **Jenkins version**: [CloudBees Jenkins Enterprise 2.46.24.0.2-fixed](https://release-notes.cloudbees.com/release/1/2.46.24.0.2)
- **Job type**: Freestyle