ubuntu/centos/debian: get_linux_distro has different behavior than platform.dist

Recently cloudinit added get_linux_distro to replace platform.dist() functionality.
The behavior returned a tuple on ubuntu that doens't match the platform.dist module.

python3 -c 'from platform import dist; from cloudinit.util import get_linux_distro; print(dist()); print(get_linux_distro())'
('Ubuntu', '16.04', 'xenial')
('ubuntu', '16.04', 'x86_64')

This breaks ntp configuration in cloud-init on xenial, which checks
(_name, _version, codename) = util.system_info()['dist']
if codename == "xenial" and not util.system_is_snappy():
...

Also CentOS 7 behavior:
[root@c71 ~]# python -c 'from platform import dist; from cloudinit.util import get_linux_distro; print(dist()); print(get_linux_distro())'
('centos', '7.5.1804', 'Core')
('centos', '7', 'x86_64')

Debian stretch:
root@debStretch:~# python -c 'from platform import dist; print(dist());'('debian', '9.4', '')

openSuse 42.3:
sles42:~ # python3 -c 'from platform import dist; print(dist());'
('SuSE', '42.3', 'x86_64')