Add a new blocking argument to register_all_nodes

Currently, if the nova-baremetal and ironic services are not
available when registering a node, the function calls will sleep
for ten seconds. Make this behaviour configurable by use of a
blocking flag that default to True.

Change-Id: I87b05acf61ef7424243cb040e10ab84a73a68ec3
Merge "Add a new blocking argument to register_all_nodes"