[0.51.0~rc1] missing dependency for pm-graph binary

p-p-c 0.51.0~rc1~ppa~ubuntu18.04.1 introduces new test cases based on pm-graph binary.

pm-graph is written in python2, but relies on configparser which is not part of the standard python2 library (it appeared in python3.2). A backport package, python-configparser, has been created to handle this.

However, this package doesn't seem to be set as a dependency of pm-graph package, resulting in errors like this when trying to run the jobs in Checkbox on a fresh 18.04 install:

------------------[ Resume from idle by using Intel pm-graph ]------------------
ID: com.canonical.certification::stress/s2idle_pm-graph_30
Category: com.canonical.plainbox::stress
... 8< -------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/bin/sleepgraph", line 63, in <module>
    import configparser
ImportError: No module named configparser
------------------------------------------------------------------------- >8 ---
Outcome: job failed

Workaround
==========

Manually installing the python-configparser package fixes the issue.