DRAC wsman_{enumerate, invoke}() to return an ElementTree object

Before the method Client.wsman_enumerate() from the DRAC driver was
returning a list of XMLs, this commit changes that behavior and instead
of returning a list it will now merge all the XMLs into one and return
the result ElementTree object to facilitate the manipulation of that
XML on the other end.

Related: blueprint drac-management-driver
Change-Id: I0e662794afe933ef57268aba975aec1fbed70846