Check if boot device is persistent on ipminative

The commit 94433a2ab522d8b6ae2e2cf24de0ff723e491c45 to pyghmi added
'persistent' to the return dict of get_bootdev(). This commit is just
making use of it.

Change-Id: I724246de951c1ac94a001ebd8c83f8ef4ffa7e66
Merge "Check if boot device is persistent on ipminative"