I recently built a ramdisk for IPA and was confused by
the fact that the source-repositories name did not
match the element name. (this is a convention,
confusing when they don't match but certainly not
required).
This patch makes it so you can use DIB_REPOREF_ironic_agent to
customize the IPA ramdisk sources when building ramdisks.
For backwards compat if DIB_REPOREF_agent is set it automatically
sets the new DIB_REPOREF_ironic_agent to that value as well.
Change-Id: I082d989d0d85601f5984dc7c3767b8d66a3d5438
