Fix gitolite installation path in gitlab:env:info

(related to #2480 and #2503)

This tasks presume of a fixed path for gitolite, relative to repos_path.
info.rake uses "#{Gitlab.config.gitolite.repos_path}/../gitolite/src/VERSION"

However, you can install gitolite wherever you want.
Hence this patch, adding a new setting in gitlab.yml.
If the user doesn't add it, gitolite installation path will be the same
than before.

M config/gitlab.yml.example:
  Add 'install_path' in 'gitolite' section
M lib/tasks/gitlab/info.rake:
  Uses '"#{Gitlab.config.gitolite.install_path}'
  If undefined, defaults to '{repos_path}/../gitolite/src/', as before