Add empty id_rsa.pub for nailgun settings template

Commit 5106d31ee80a683c618fada7b0186263e1d20a28 adds a
nailgun dependency on /root/.ssh/id_rsa.pub, but docker
build fails because it doesn't exist during this phase.

Change-Id: Ib36adcebe188627ed62b24b81e708edafd52cdaf
Closes-Bug: #1392698
Merge "Add empty id_rsa.pub for nailgun settings template"