Chassis status monitor class

Create a chassis status monitor class that can be used by multiple
applications in this repository.

Support monitoring the following chassis properties:
* Present
* Ava

Chassis status monitor class

Create a chassis status monitor class that can be used by multiple
applications in this repository.

Support monitoring the following chassis properties:
* Present
* Available
* Enabled
* Last requested power state
* Power good (pgood)
* Chassis input power status
* Power supply power status

Specify which properties to monitor when creating an instance of this
class.

Listen for the following D-Bus signals for these properties:
* NameOwnerChanged
* InterfacesAdded
* PropertiesChanged

Tested:
* Ran automated tests
* Manually tested all methods in simulation:
* Tested getting all properties when class is created
* Tested getting properties in NameOwnerChanged callback
* Tested getting properties in InterfacesAdded callback
* Tested getting properties in PropertiesChanged callback

Change-Id: I3558b98769525cfedc44361eb6fccb2bccd74a40
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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