Fix insufficient vnodes in verify_listkeys

Fixes issue #149, related to basho/riak_core#228
If queried right after it has joined the cluster, a node may not have
the other nodes registered by the node watcher yet. Switching to waiting
until all nodes in the cluster have been registered by the node watcher
before performing query.
Also, querying for all nodes after stopping/starting just in case,
although I haven't seen problems there, but sound reasonable.
Removing assert from the call to wait_until_no_pending_changes since it
is already asserting internally.
Merge pull request #149 from basho/eas-fix-insufficient-nodes-listkeys

verify_listkeys failure