Add testcase for multiple discovery-driven sources

This test was intended to reproduce the reported issue around
resources discovered for one pollster bleeding into the resource
list passed to an unrelated pollster with the same interval.

However the observed behaviour seems correct.

Related-bug: #1335986
Change-Id: Ifb608246ad2ee2d858b4268ae2e8dc6faaff025c
Remove redudent space in doc string

In file ceilometer/compute/plugin.py, there are some redudent
spaces, the patch will remove them and make the code more pleasure:)

Change-Id: I1a896366bab3344422d905b30b24cbb8f257ddde
Merge "Remove redudent space in doc string"
Merge "Add testcase for multiple discovery-driven sources"