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
81cf99a5

.

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>