Give ssh-keygen SELinux rights to read tempfiles

This commit adds an SELinux module for Centos 7 that enables ssh-keygen
to read the temporary files GitLab creates for it.
Document the gitlab-selinux README
Merge branch 'gitlab-selinux' into 'master'

Give ssh-keygen SELinux rights to read tempfiles

Thanks to @tinuva for finding the solution [1] to this problem and to @axil
for his nice blog post about GitLab and audit2allow [2].

[1]: https://github.com/gitlabhq/gitlabhq/issues/7413#issuecomment-53181049
[2]: http://axilleas.me/en/blog/2013/selinux-policy-for-nginx-and-gitlab-unix-socket-in-fedora-19/

See merge request !193