Install Odoo 12 and Wkhtmltopdf on Ubuntu 18.04 or Debian 9

Install Odoo 12 Community. We can install Odoo 12 community on Ubuntu 18.04 server or Debian 9 server in the same ways. Odoo 12 use Python 3 and has better performance than before. There are several steps we need to do, so that Odoo can run properly.