Move wwwroot51 credentials to webserver-alkane role

Summary:
This mechanism of deploy keys to clones repositories can be useful
to access private repositories in Alkane.

One use case is Zed user content, which is currently a private repo.

Ref T1676

Test Plan: Deploy Zed

Reviewers: dereckson

Maniphest Tasks: T1676

Differential Revision: https://devcentral.nasqueron.org/D3187