NEXUS-6182: Added sync to Image strategy too

not that it matter, as this mojo might be run without
project only...
NEXUS-6182: Initial sleep imple

As the "last" definition is okay (last when topologically
sorted), but, in case of parallel build given last might be
executed with some others assigned on same level. As the
module build durations might be wildly different (ie. some IT
running project vs some last POM-only one), the added code
will simply "hold" (by sleeping) the current thread until every
other project in the build gets some build result.
NEXUS-6182: REshuffled logic

As in parallel builds it's needed to wait for all, and then
evaluate any possible problem in the build.