install: make yum install more robust

If a yum install fails, try a few more times just in case it's a
transient error due to a mirror problem.
install: copy rsync bind key to root folder of web server

Similiar to how the SSH key is copied for the nodes, allow the bind key
to be retrievable by the other brokers.
install: have node fetch key from broker indefinitely

Installing the broker key is the last step of the node configuration.
Since the broker is able to complete setup without needing the node to
reboot beforehand, make this key retrieval step continue until success.
Given that if the broker doesn't come up the install has failed anyway,
this seems like a safe change.