add edx-west/.gitignore for symlink to config repo

Not checking those symlinks in since they are symlinks to another repo,
but generally people will want these.  On my machine they look like
this:
    configuration-secure@ -> ../../../configuration-secure
    edx-secret@ -> ../../../configuration-secure

Just for the edx-west configuration files.

playbooks/edx-west/.gitignore

+configuration-secure
+edx-secret