Add certificate lifetime configuration option

[osirisinferi]

An alternative to #353, probably not necessary to add both 😛

[aarongable]

Of the three mechanisms you've proposed for making this configurable, I like this approach by far the best. Just a few small comments (nothing substantive, mostly naming nits and slight simplifications) and this'll be ready to go!

[osirisinferi]

Hm, not sure if you'd see my latest comment above, so I'll repeat it here:

time.Duration() uses an int64 internally which will overflow the NotAfter date if the duration is set to 9223372038 or more (the floored max. of an int64 plus 1 second (because we do -1) and an extra second to overflow it.

Which results in this:

        Validity
            Not Before: Aug 23 19:47:10 2021 GMT
            Not After : May 14 19:59:54 1729 GMT

[andygabby]

Thanks!