dbus usage

The following are guidelines for bmcweb's use of DBus to construct Redfish schemas:

• Do not require (ie set 500 internal error on) an implementation present a property that wasn't in the first commit of the interface unless it can be verified that all OpenBMC implementers of the interface support that property.
• If an implementation presents a property that doesn't match the type specified by the interface at any version, it shall be reported as a 500 error.
• If the DBus interface definition has an "unknown" or "unspecified" value, detecting these will omit the property from the Redfish tree, in line with the Redfish specification.
• All DBus interfaces on all object paths are optional. An object missing an interface is never an error, and shall simply omit the relevant properties and/or actions from that Redfish Resource.
• bmcweb will code to the DBus interface itself. This means that daemons are expected to handle functionally bad, but DBus-correct input in their own process, and return appropriate return codes. This is done to reduce the duplication in input processing between the various user-facing daemons.
• There are interfaces for which there is an expectation that there will only ever be one producer in the project (ex bmc networking, user management). In these cases, it is desirable to call the daemon by well known name directly. Bmcweb APIs should call the mapper in cases where it's reasonably expected that multiple implementations exist (ex, CPU management, Sensors).