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.
install: Bypass RHEL specific repo configuration

Just rely on default CentOS yum configured repos.
install: add detection for CentOS

Modify preflight check to parse CentOS release string properly.
install: copy bind key to web server

This is a change that would not be suitable for production, but removes
a step of copying the key manually later. With the key available from
the primary broker's webserver, the other brokers can download it easily
and configure their DNS settings.