Support configdrive from HTTP(S)

This commit adds support for passing an HTTP(S) URL as the
`configdrive` parameter for the `prepare_image` command.

This will load the entire image in memory. Although the configdrive
is limited to 64MB, this is not ideal and should be made better
in the future (likely when imaging gets refactored).

Change-Id: I6f14b9059920d2cef108c30d027c5486fa0b7e9a
Implements: blueprint expose-configdrive