[EWG] - Can now configure any line in nexus.properties...

...by setting the appropriate key in the 'nexus_properties' dict

[2] Also fixed an issue whereby we weren't setting the permissions and ownership on the installation directory correct (we were instead setting the permissions on the downloaded archive).
[3] Also set the nexus user's default shell to '/bin/bash' as it is required to log in as this user to start Nexus.

The behaviour of setting keys and values in ``nexus.properties`` is now slightly different, because it is now a dict the whole dict gets replaced if the user re-specifies the dict outside of the role defaults. I have updated the README to explain the new behaviour. It felt a little counter-intuitive to me at first as I'm so used to Hiera's hash merging behviour, but given that Ansible doesn't default to dict merging I think it's reasonable. What do you think?