EnvironmentMetrics: Adjust PowerLimitWatts functions for reuse

Work is in-progress to add PowerLimitWatts support to the
EnvironmentMetrics property for both Processors[1] and Chassis[2] paths.

The Processors commit adds functions for getting the object implementing
the xyz.openbmc_project.Control.Power.Cap interface. It uses the
controlled_by association to find the object.

That commit also adds a function to fill in the PowerLimitWatts
properties for the GET response.

These functions can be used for the Chassis path as well.

Additionally, the Chassis path adds setting of the SetPoint and
ControlMode properties of PowerLimitWatts. For the PATCH path the common
function can be used to get the object with difference in error
handling.

In order to allow for the reuse this drop is adjusting these functions:
 - Added common PATCH handling functions
 - Renamed getPowerLimitWatts -> getPowerCapObject
   This is the function that gets the associated object implementing the
   Power.Cap interface. This can be used by either the GET or PATCH
   path by specifying a different callback. Adjusted to pass errors to
   the callback for handling. Except special-case of internal error
   where multiple objects are returned as controlling the object.
 - Renamed getPowerCapFromControl -> getPowerLimitWatts
   This is the function used for GET to fill in the PowerLimitWatts in
   the response.

[1] https://gerrit.openbmc.org/c/openbmc/bmcweb/+/87018
[2] https://gerrit.openbmc.org/c/openbmc/bmcweb/+/47300

Tested:
 - Tested with commit [2] using Chassis path

Change-Id: I8272fb9ab80e8063858bb2016dc209a1ca0b2038
Signed-off-by: Janet Adkins <janeta@us.ibm.com>