Cross-platform udevadm_settle

RHEL doens't have /sbin/udevadm. Call /sbin/udevsettle instead.
Use native method instead of shelling out
Fix typo in comment
Use df's POSIX output format

so that a mount point doesn't go to multiple lines
Prefer Bosh::Exec when shelling out
Merge 'origin/master' into rhel-agent

Conflicts:
	bosh_agent/lib/bosh_agent/message/disk.rb

Change-Id: Ic3afa9761a4fa433e11155adabb05950c9946365
Merge pull request #35 from anfernee/rhel-agent

Some necessary changes that make agent runnable on rhel