Fix sample-create in v2 api calls

A dict embedded in a list is the expected input. In
47934c777c50897b649793e0960eebdaad322c45 the list was lost, this
puts it back.

Note that the _exact_ date of this failure was identified by
existing Rally jobs which started failing in early August. That made
searching the commit logs straightforward.

Change-Id: I37e794de7a23b8ce8e67a086732af5a19f182409
Closes-Bug: #1367867