Update gnocchi for centos binary

Gnocchi packages are now available in centos repos[1]
for liberty and the images build.

backport: liberty

Closes-Bug:#1478145
[1] http://mirror.centos.org/centos/7/cloud/x86_64/openstack-liberty/

Change-Id: I957754d4a4e35b026d3e27ee23f33ce2e78592ec