Allow to overwrite location of gitlab.yml config file.
Allow to specify secret token via environment variable.
add steps to monthly releases
Update installation guide with new shell version.
remove redudant text in release doc
Merge branch 'detailed-monthly-release' into 'master'

Detailed monthly release

Fixes #1190
Merge pull request #6783 from pkgr/allow-secret-token-env

Allow to specify secret token via SECRET_KEY_BASE environment variable.
Merge pull request #6782 from pkgr/allow-to-change-gitlab-config-location-via-env-variable

Allow to overwrite location of gitlab.yml config file.
Merge branch 'shell_version_install_doc' into '6-8-stable'

Update Shell version in the Install doc
Merge branch 'shell_version_install_doc' into 'master'

Update Shell version in the Install doc
Merge branch 'master' of https://github.com/gitlabhq/gitlabhq
version 6.8.0
Merge branch '6-8-stable' of https://dev.gitlab.org/gitlab/gitlabhq into 6-8-stable