Make Agent availability help more flexible

The help of the agent availability (retention strategy) is hard coded into Jenkins as html files. This has 2 drawbacks:

• Not all translated pages have translations for all possible availability options (partially translation is there but only as comment in html), the description is then missing.
• Additional retention strategies provided by plugins can't provide their help

The proposal is to have the help generated in the same way is it is done for the launcher, so that each strategy can define it's own help files including translations.

This would also show english as fallback, in case a translation is not available.