Update dev quickstart guide for ephemeral testing

Devstack now supports the IRONIC_VM_EPHEMERAL_DISK variable
which allows the user to specify the size of the ephemeral
disk partition. This change documents how to use this variable
to boot an instance with ephemeral storage.

Change-Id: Idd3bf8c4c63d32a8ed2bacd37ba642eb70f709db
Merge "Update dev quickstart guide for ephemeral testing"