luci-base: set the fallback default of rollback timeout to 90s

luci-base: set the fallback default of rollback timeout to 90s

Set the fallback value of the config change rollback timeout
to 90 seconds to match the change in /etc/config/luci by commit
81cf99a50.

That commit changed the value in the config file, but did
not change the underlying fallback values that do get applied
when there is no proper config item in etc/config/luci.

Users sysupgrading from old systems may have carried an ancient
/etc/config/luci (without rollback config) with them, so this
change should help them to see the intended user experience.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>