Stop creating a swap partition when none was specified

Formerly, when the Nova flavor specified no swap partition, Ironic would
still create a 1mb swap partition to avoid certain issues. Those have
been fixed in previous patches. This patch corrects the behavior such
that Ironic will only create a swap partition when one is requested.

Closes-Bug: #1273785
Change-Id: I6f1c2de3fc1128fd92911fa5ffc737eb20a17409
Merge "Stop creating a swap partition when none was specified"