Allow the wait_for_entropy callback enforce continue

This way e.g a timeout or a way allowing user to ignore the lack of entropy can
be implemented.

Also document the potential issue of waiting for enough entropy for a long time
or even forever.

Related: rhbz#1073679
Pack data for the wait_for_entropy callback

Packing data into a namedtuple is good for keeping the API even if we need to
add some more parameters (fields) in the future. It also serves as a
documentation.

Also improve the documentation of the callbacks that are expected to be used.

Related: rhbz#1073679