Generate a unique name for each vbucketmigrator command.
Generate a unique name for each vbucketmigrator command.

This needs to be globally unique within the cluster because the
vbucketmigrator arguments are generated on whatever node is running
the orchestrator, and we don't want to accidentally reuse them if the
orchestrator moves.

Change-Id: I02267e25769cf991fde5ad1e68c146572061ac23
Reviewed-on: http://review.membase.org/4702
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,membase/ns_server,daverigby/ns_server,membase/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,daverigby/ns_server,nimishzynga/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,vzasade/ns_server,mschoch/ns_server,vzasade/ns_server,membase/ns_server,pavel-blagodov/ns_server