Handle race for the already existing layer
Handle race for the already existing layer

Hard copying a large layer takes time.
Renaming of a blob may also be not instant for some cases.
Handle race for the already existing layers by skipping competing
layers symlinking or renaming blobs made by concurrent workers

Change-Id: I5fb83a13e385ed1a35a6cb735e50df2b5b368164
Closes-bug: #1864953
Signed-off-by: Bogdan Dobrelya <a2228ae37d00d1cbd5a93e53f3e1e5aef21d5900@redhat.com>

openstack/tripleo-common,openstack/tripleo-common