Change from live to on time in background computation

[registerrier]

This PR changes the calls to make_map_background_irfto take ontime i.e. (TSTOP-TSTART) rather than livetime (i.e. ontime corrected by dead time fraction) to follow the specifications presented in:
https://gamma-astro-data-formats.readthedocs.io/en/stable/irfs/full_enclosure/bkg/index.html#notes

docstring is changed as well as the call in MapMakerObs. Tests are updated accordingly.

[cdeil]

@registerrier - Thanks!

You use "ontime" here, but on the events and obs container the property is called "observation_time_duration".

We should probably add a section below http://docs.gammapy.org/0.8/development/howto.html#coordinate-and-axis-names where we define names to be used consistently throughout Gammapy, i.e. in the obs classes, events classes, and functions like this one. My suggestion would be to just use "observation_time" for this case, because it's a little more intuitive than "ontime"? But either way is OK for me, I just don't like the mix. @registerrier - Could you open a separate PR with a short section and suggestions for names? (I don't think we need to stick to the current names on the obs and events classes, we're allowed to make changes and clean and better classes once now.)
And then implementing the renames for consistency can be done one PR at a time in the coming weeks, e.g. @dcfidalgo could do it for the obs classes; trying to discuss and decide the names, and editing many places in the code wouldn't be pleasant.