use configurable auto responsive credentials provider for git server to ...

...allow to be used in background tasks where user interaction is not possible

this pull request aims at providing configurable way of use of default credentials provided used by jgit when interacting with git server. This is a required step to allow to use git server from background threads where interaction with user is not possible so it does allow to directly accept ssh key for given host otherwise (current behavior) it will simply fail as user/system could not reposed properly to prompt issued by ssh server.