Add support for fetching a role from a repository that is stored in the root of the repository

In order to install roles outside of our control, where the role is stored in the root of the repository, we need to be able to specify that the role is stored in the root of the repository and a name to give that role once we've cloned it.

Therefore, I have added some additional optional variables to the gitsource role source - to specify that the repository is a single role itself, and the name to give that role.

Context

I would like to install the role: https://github.com/observIQ/bindplane-agent-ansible , but the role itself is just in the root of that repository.