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: have node fetch key from broker until timeout

Instead of trying to download the key once and move on, the wget attempt
is repeated for a set amount of time. Once the key is downloaded it is
checked to be valid before inserting into the authorized_keys file.
install: make yum update more robust

If a yum update fails, try a few more times just in case it's a
transient error due to a mirror problem.