Feature/fixes

Thanks for this great role.  Really helped me install Instant Client quickly.

I've made a couple of tweaks:
- Added oracle_client_install_cx_oracle variable to control install of cx_Oracle (default: true).
- Added oracle_client_local_oracle_zip_files_dir variable to specify local location of instant client files.
- Changed role to add oracle user and oinstall group if needed
- Added a CHANGELOG.md file