Allow to set up custom ids for authN headers

[alberto-brigandi]

Right now the header id is inferred from the type of the IaaS on which the user wants to authenticate (e.g. "ost" for OpenStack or "one" for OpenNebula); it's a nice feature but it also prevents to send in the header authN info for multiple IaaS of the same type (as there would be multiple line with the same id).

This can be resolved either:

• overloading the static method buildCredentials() passing the desired id as parameter
• adding a withId(String id) method that overrides the default header id

[alberto-brigandi]

Hi @alpegon,
I wrote a possible fix for this issue; unfortunately seems like the withId(String id) path can't be followed, as the credential properties are added in chain and thus no replacement of the previously set id can be made