RHEL Disk resizing and Parallelized converge

Preparing for a new round of load testing, two new features for ec-metal:
1.  Fix root disk resizing on newer RHEL/CentOS AMIs, which requires a reboot.  If you want to hate on Redhat for this, channel it to: https://bugzilla.redhat.com/show_bug.cgi?id=1155742
2.  Because it's been annoying me forever,  I am parallelizing the converge of all non-bootstrap EC nodes.