Sleep between fdisk runs

On a VMWare Workstation VM doing fdisk back to back results in a "device
busy" error. Putting a sleep between fdisk calls lets it succeed.
Put quotes around ${stage_uri} null comparison

Otherwise it complains about the stage_uri string not being a binary
operand.
Fix stage3 tarball regexp

The existing regexp matched multiple lines in latest-stage3.txt. Fix is
to only match a single line.
Merge pull request #7 from mrpdaemon/sleep-between-fdisk

Sleep between fdisk runs
Merge pull request #8 from mrpdaemon/fix-stage3-fetch

Fix stage3 fetch