Completely subsume POWER_MANAGER into testenv

POWER_MANAGER is currently pulled out of the JSON datafile when
building the undercloud, which means we don't need to pollute the
environment with it. Pull it into devtest_testenv, and no longer write
it out in tripleorc.

Change-Id: I053e0e085a3243e6b1283712fc3fc0897ed040b6
Merge "Completely subsume POWER_MANAGER into testenv"