Newer EC2 instances are both cheaper and have generally better
performance.
The dnsmasq configuration had a hard-coded interface name, so in order to
accomodate instances with more recent networking that result in so-called
predictable interface names, the dnsmasq configuration needs to be replaced at
runtime with userdata to select the default interface.
* remove outdated references to envchain in documentation
* add new host volume locations in userdata
* don't exit the entire script during provisioning, just return