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.