convert gds-ssh-config to use concat
create separate ssh_config module

This new module allows separate ssh config fragments to be
independently managed. That way people with responsibilities in
multiple teams can manage their ssh config independently.
split extra ssh config out of gds-ssh-config module
Add comment explaining inconsistent Puppetfile syntax for concat
[alexmuller] Add personal SSH fragment
[alexmuller] Use a file rather than template

Thanks @philandstuff.
Merge pull request #29 from alphagov/ssh_config_concat

Manage ssh config in separate pieces