[rbd-target-api] Add Ubuntu to read_os_release dict

Environment:
Ceph-iscsi from master branch
Ubuntu 19.10
Using python3

When running Ubuntu, I get some of the requests failing with:
{
  "message": "MACHINE_NAME failed package validation checks - OS is unsupported"
}

Looking into the code, I can see method read_os_release is called and Ubuntu is not on the list. Ubuntu 19.10 offers almost all base packages for ceph-iscsi installation using setup.py. Therefore, I would suggest minimum version to be 19.10.

Start by locating the read_os_release method and the package validation checks that report Ubuntu as unsupported. Review how supported operating systems and minimum versions are represented, then verify that Ubuntu 19.10 is accepted without breaking existing validation behavior.

Written by the indexing model from the issue text.