edxapp-git-auth: allow authenticated edx-platform/edx-theme checkouts

@jarv This should allow to use ssh auth conditionally on edxapp, for both the edx-platform and edx-theme repositories checkout, depending on the `EDXAPP_LOCAL_GIT_IDENTITY` variable. It's based on the same approach as in the certificate and xserver roles, ie a temporary deployment of a private key loaded through GIT_SSH.

Ticket: TKTS-799