Revert "Switch hpcloud devstack-trusty to dib image"

This reverts commit 073df3c78366477b6f15b866e4854cf37287a41d.

Nodepool currently assumes that each label is provided by a singular
image build source. This is not true for our dib situation currently as
devstack-trusty was expected to be provided by dib on hpcloud and via
the old school VM snapshot build method on rax. This expectation was
broken by nodepools single label build type ability.

Work around this by going back to old build method for devstack-trusty
everywhere.

Note this updates the HPCloud trusty base image to the current value.

Change-Id: Iffba739dd335de853ef90b6c37a8dc034932003b
Merge "Revert "Switch hpcloud devstack-trusty to dib image""