Set default setting common to all projects

Configure the merge type to fast-forward and ensure merges can be done only if the pipelines are succeeding.

Some examples:

./gitlab-rulez diff rulez.yaml --filter infrastructure/gitlab-rulez
infrastructure/gitlab-rulez
  Set default settings
    settings
    - only_allow_merge_if_pipeline_succeeds="False"
    + only_allow_merge_if_pipeline_succeeds="True"
    = merge_method="ff"
  Set unversioned branches
    protected_branches
    = name='master', merge_access_level='30', push_access_level='40'
    default_branch (deprecated: use settings:default_branch:$foo)
    settings
    = default_branch="master"
settings: project infrastructure/gitlab-rulez: set only_allow_merge_if_pipeline_succeeds to True
settings: project infrastructure/gitlab-rulez: save changes
computed 2 actions

$ /gitlab-rulez diff rulez.yaml --filter pkg/development/w3m
settings: project pkg/development/w3m: set only_allow_merge_if_pipeline_succeeds to True
settings: project pkg/development/w3m: set merge_method to ff
settings: project pkg/development/w3m: save changes
ensure_branch: project pkg/development/w3m: branch apertis/v2020dev0 from apertis/v2019pre
protected_branches: project pkg/development/w3m: add "apertis/*"
protected_branches: project pkg/development/w3m: add "debian/*"
protected_branches: project pkg/development/w3m: add "pristine-lfs"
protected_branches: project pkg/development/w3m: add "upstream/*"
protected_branches: project pkg/development/w3m: drop "apertis/v2019pre"
settings: project pkg/development/w3m: set default_branch to apertis/v2020dev0
settings: project pkg/development/w3m: save changes
settings: project pkg/development/w3m: set ci_config_path to debian/apertis/gitlab-ci.yml
settings: project pkg/development/w3m: save changes