Restrict sort_keys for stack lists

We are only allowing users to sort stacks on a subset of columns that
are known to be indexed.  Currently allowed sort columns are:

created_at
updated_at
name
status

Change-Id: Ib4f955336edd7d86a1b6bf98e38378b0d495d0dd
Closes-Bug: 1252116
Merge "Restrict sort_keys for stack lists"