Do not regenerate ironic ssh key if one exists

Gets rid of redundant globals and avoids regenerating an ssh key
for ironic if one exists.  Also allows key file to be user-specified,
letting ironic use pregenerated keys.

Change-Id: Iaa6f8918bf14dae2116269a0be5039fc5afaecfa
Merge "Do not regenerate ironic ssh key if one exists"