Backport #232 and #282 to 0.9.x

In order to move to a more reliable automated testing framework, we need to have control over the graphite root prefix. The fixes in #232 and #282 add the ability to set an envvar that disables the `[install]` section in setup.cfg allowing for tests to be run without requiring write permissions to `/opt/graphite`.

/cc #219 since this brings us closer to a more permanent fix.