Allow teams::infrastructure users to disable git config
Allow teams::infrastructure users to disable git config

@nickstenning pointed out that some people manage their ~/.gitconfig
directory themselves and so a git::config::global resource will
interact with that file resource in bad ways.

This commit adds a parameter to allow a user of teams::infrastructure
to disable its management of git config.

alphagov/gds-boxen,alphagov/gds-boxen,alphagov/gds-boxen