Python 3 only on the default installs

See https://docs.google.com/spreadsheets/d/1PnPLZRM2hu7ucv1yuq7Fd5hjNw_DYupHIYZpoP4-xHk/edit?pli=1#gid=1918227221 for the initial set of packages needing attention for desktop

See also the list of tagged bugs: https://bugs.launchpad.net/ubuntu/+bugs?field.tag=python3

Work items:
[robert-ancell] Drop software-center: DONE
[robert-ancell] Look at using oauth directly instead of ubuntu-sso-client: TODO
[robert-ancell] Drop ubuntu-sso-client-qt as a depends of ubuntu-desktop: TODO
[doko] Drop landscape-client-ui-install; it's unsupported: DONE
[mterry] Drop deja-dup dependency on duplicity, and prompt for install if required: DONE
[barry] Contact Xapian upstream and Debian maintainer about 1.4 release with supported Python 3 bindings: DONE
[barry] Port apt-xapian-index to Python 3: DONE
[barry] Contact samba maintainers, re: Python 3 bindings (python-talloc): DONE
[vorlon] Work on patch to install samba-common-bin on demand for nautilus: DONE
[vorlon] Contact tkamppeter re: system-config-printer port (Got actually contacted by Will Cooke, system-config-printer is already switched over to Python 3. tkamppeter): DONE
[tkamppeter] Demote python3-smbc to Suggests in system-config-printer (LP: #1552868): DONE
[smoser] Drop/fix cloud-image-utils need of python2: DONE
[smoser] Remove the cloud-image-utils dependency on euca2ools: DONE
[doko] Rebuild vim for Python 3 only: DONE
[zyga] Make checkbox use only Python 3: DONE
[barry] Look into LP: #1440504 to split the dependency of libpeas from libpython2.7: DONE
[barry] Drop Python 2 dependencies from oneconf-common (LP: #1440375): DONE
Drop python 2 dependencies from postfix (LP: #1538198): DONE
[barry] Drop Python 2 dependencies from xdiagnose: DONE