(maint) fix filename for acceptance sshkey

The boostrap script `acceptance/bin/ci-bootstrap-from-artifacts.sh` uses
an outdated filename/SSH key for our acceptance runs. We've been
(temporarily) hacking around this by sed'ing this file in our Jenkins
jobs. This commit fixes the filename in the script itself.
Merge pull request #3289 from rji/maint/stable/acceptance-sshkey

(maint) fix filename for acceptance sshkey