[lfmerge] Don't fail if mongodb already installed
[lfmerge] Don't fail if mongodb already installed

This can happen when another build already installed mongodb. In
that case our semaphore file doesn't exist in this project, so
simply ignore the failure.

Change-Id: I61fb9e938338901a21be27e6de62cff43e3b5a96

sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts