bundlespec: do not overwrite bundlespec value with the config one This is finally making the `obsolete` bundlespec paramater work.

bundlespec: add a `overwrite` parameter to set_param This will open the way for the configuration value to yield in front of the explicit bundle type.

bundlespec: merge the contentopts and params dictionnary They are content using the same keys. Using differents object for access open the gates for confusion in the code using them (this is already the case). So we start fusing their usages to make the parameters more useful. More work will be needed to make them really useful, but the first step is here: not throwing the value away. However this is still not making the previously introduced test useful because currently, the default config value overwrite the one from the bundlespec. We will fix this in the coming changesets.

bundlespec: test if the "obsolete=" parameter is read Narrator voice: Actually it is not. We will fix it in the next changeset.

bundlespec: fix lack of title in a evolve tests This is a full new test case and should be "flagged" as such.

bundlespec: make the `stream` case less special The handling of the stream case seems fragile (does not account for newer parts and options that will arise) and has special code dedicated to it. To simplify and strengthen things, we make it use the same mechanism as the other options. So we make it less special by making it a special value in the common case.