clouds.yaml is read many times

[samuelallan72]

NewExporter and NewServiceClient each call clientconfig.GetCloudFromYAML, which opens and reads the os-client-config (clouds.yaml). There are two (somewhat minor) problems with this:

• These functions are called once for each service every time metrics are collected. This results in a non-negligable performance hit (~2% total cpu time taken in this function).
• If clouds.yaml is changed on disk while the exporter is running, the changes will be re-read. This is unexpected, and could cause crashes if the file is saved partway through being edited.