Use default as name for proposals of barclamps with a unique proposal

While the name here is not localized anymore, the rationale is that this
is the same name as used during the install process for core crowbar
barclamps. This makes things more consistent. Also, the name is much
less visible now if multiple proposals are not allowed.
Merge pull request #535 from vuntz/name-unique-proposals

Use default as name for proposals of barclamps with a unique proposal