Add support for graphite-api

Update module to be compatible with both graphite-web and graphite-api.
Use sprintf string formatting instead of concatenation

Use sprintf string formatting instead of concatenation to prevent errors
caused when the tenant ID is numeric. When the tenant ID is numeric,
PyYAML parses it as a python int type, which does not support
concatenation with strings.

Using sprintf string formatting works around this problem.
Merge pull request #1 from dlobue/graphite-api_compat

Add support for graphite-api