REST: Add DELETE support

Add support for the DELETE verb.  This verb will cause the
Delete method on the xyz.openbmc_project.Object.Delete interface
to be called on the specified object path.  An error will be
returned if that interface/method doesn't exist on that path.

The code is similar to the method handling code, except it
doesn't need to parse any argument JSON, and it is looking only
at a specific interface.  It does share the code path that
introspects the object to find a method and call it.

Tested:  Used it to delete error logs.

Change-Id: Ica90b0d80049e6bc59fe2b8456948696054f4a8b
Signed-off-by: Matt Spinler <spinler@us.ibm.com>