fix #2336: ui: adjust message for bulk start/stop/migrate

fix #2336: ui: adjust message for bulk start/stop/migrate

The message in the Task Log has been 'Start/Stop/Migrate all...',
which is misleading since not everything might be affected by bulk actions.
This also affects the messages send at a nodes startup and shutdown, but
since this just affects a subgroup of VMs/Containers (those who are
onboot=1) the new wording still applies better than the previous.

Signed-off-by: Folke Gleumes <f.gleumes@proxmox.com>