DistUpgrade: set NEEDRESTART_SUSPEND during upgrades

Set NEEDRESTART_SUSPEND to a non-empty value by default. When the
upgrade is finished, the user will be prompted to reboot, so there is no
need for needrestart to prompt the user about service restarts during
upgrade.

LP: #1968919

Signed-off-by: Nick Rosbrook <nick.rosbrook@canonical.com>