Fix git config user.name for gitlab.

rake gitlab:env:check assusmes git global config user.name must be "GitLab".
That is incorrect. It can be anyname the gitLab admin wants.

This patch add a new setting gitlab.git_user, and make sur that,
if that new setting is undefined, gitlab.git_user will default to "GitLab".
(so the current rake behavior is unchanged)

M config/gitlab.yml.example:
  Add new setting gitlab.git_user
  Mention default value "GitLab" for gitlab.git_user
M lib/tasks/gitlab/check.rake:
  Uses 'Gitlab.config.gitlab.git_user' for 'user.name'.