The AWS and Azure drivers incorrectly required the user to supply
the username in the pool configuration when using diskimages.
The OpenStack and IBMVPC drivers correctly use the top-level
diskimage configuration to determine the username.
Correct this by deprecating the pool-level configuration in the
drivers that offer it, and default it to using the top-level
configuration.
Change-Id: I4e6b4d4268b32ab7b397a11dd0ccd08b18c09a86
202230e16b