Correctly reject request to add lists of hosts to an aggregate

Attempting to add or remove a list of hosts to an aggregate
results in an internal server error. This patch fixes the input
validation so such a request is correctly rejected and a 400
error returned.

Change-Id: I760db36d685cff20b01a72160b8b3fbb8a70b412
Closes-Bug: #1321653
Merge "Correctly reject request to add lists of hosts to an aggregate"