So currently if some optional openstack service is not installed
metadata will fail when it tires to work on its config file.
This commit adds in a check for the existance of each service config
before running the gathering tasks.
What might be a useful addition to this commit would be some logic
for determining which services should never be missing and alerting
the user.
Change-Id: Ie360f062646cb5431de9ed21bd0dd193b84ddcda
cddac5f1e9