Rename list_updates to list_upgrades

In preparation for #3210, rename list_updates to list_upgrades. Also,
link list_updates to list_upgrade so that people used to running "zypper
list-updates" from the command line will still see the available
upgrades.

Hide list_updates from sphinx docs.
Merge pull request #3251 from archtaku/list_upgrades

Rename list_updates to list_upgrades