sdbusplus: Migration from mapbox::variant to std::variant

[William Kennington]

Now that openbmc c++ projects are moving to c++17, we can finally
consider using std::variant in place of mapbox::variant. My goal with
this transition is to eliminate breakage by performing the migration
in a number of steps, working with all of the projects using
sdbusplus. I'll post updates to the mailing list as we progress
through the stages of this change, so out of tree users are aware of
the changes.

The outline is as follows:
  - Convert all users who hardcoded references to mapbox::variant over
to the sdbusplus::message::variant and sdbusplus::message::variant_ns.
  - Modify the mapbox::variant inside of sdbusplus to provide
interfaces which match
  - Move all users of non-standard mapbox variant interfaces to the
superset provided by our mapbox::variant and std::variant
  - Redirect the sdbusplus::message:variant_ns namespace alias to std.
This will roll all packages over to std::variant.
  - Fix all references in our packages to use std instead of
sdbusplus::message::variant_ns and sdbusplus::message::variant
  - Remove sdbusplus::message::variant_ns and sdbusplus::message_variant

We are currently in the first step of removing all hardcoded uses of
the mapbox::variant.

Best,
William