[#52858717] setting up ssh key for jenkins

Jenkins user needs the keys so it can connect to github and github.gds
We are placing the keys in a directory that suits the master, but am
hoping that we can parametise it later so it will work for the slaves
also
[#52858717] gen jenkins ssh key on ci-master
Merge pull request #18 from alphagov/gen_ssh_key_for_jenkins

[#52858717] setting up ssh key for jenkins