default.xml: update to YP 3.1.5

default.xml: update to YP 3.1.5

Update poky to the tagged yocto-3.1.5 revision, and also uprev the
meta-openembedded, meta-python2, meta-virtualization, and
meta-iot-cloud layers to their latest dunfell revisions to pick up
fixes.  The reference hardware BSP layers have not been updated, as
they are already up to date with upstream.  Updates to community
supported hardware BSP layers will be addressed separately at a
later time.

Bug-AGL: SPEC-3793

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I0edda19a6a70c1927f6a05e56f4894adcf8d0153
(cherry picked from commit 2ddb59fc39f6ba21c83a720f1c0f49ca0b084721)