mgr: "predicted failure" -> "life expectancy", and as a range

mgr: "predicted failure" -> "life expectancy", and as a range

1- Change terminology from "predicted failure" to "life expectancy", which
has a more intuitive meaning.

2- Change from an expected time stamp to a range.  Any expectancy has an
inherent uncertainty, so a single time is not meaningful.  Instead, express
as a range, e.g. "4-6 weeks from now", where we record the min and max
date as well as when the prediction was made.

This is still a bit awkward, but I'm not sure what is better.  It will
always be a bit awkward to express uncertainty since in precise terms it
is probably a 90% confidence interval or something.

Signed-off-by: Sage Weil <sage@redhat.com>