We encountered bug 2037312 in unit tests when attempting to get this
change rolled out. Heat apparently will attempt to set is_admin using
policy logic if it's not passed in for a new context; this breaks as the
context we are requested doesn't have all the needed information to
exercise the policy logic.
is_admin is just a bool; it's not sensitive; easiest route forward is to
add it to the safe list
Closes-bug: 2037312
Change-Id: I78b08edfcb8115cddd7de9c6c788c0a57c8218a8
c1b606f77e