Added the ability to specify checksum for etcd
Added the ability to specify checksum for etcd

It is possibe for the user to override te etcd version, thus download a
different etcd file, but the checksum is constant, so the checksum
verification will fail in that case.
Added the ability to specify a different checksum, so the user would be
able to specify the new version checksum

Change-Id: I85af3af841ae957964f18d4e37a86ab0703882bc
Closes-Bug: #1736718

nawawi/openstack,nawawi/openstack,noironetworks/devstack,openstack-dev/devstack,dtroyer/devstack,openstack-dev/devstack,group-policy/devstack,group-policy/devstack,noironetworks/devstack,dtroyer/devstack