Fix bindep platform declaration
Fix bindep platform declaration

Bindep takes a list of platform identifiers and profile names.
Each platform needs to be prepended with "platform:" to mark
it as such, with any other stand alone word being interpreted
as a profile name.

This fixes and error where we tried listing multiple platforms
in one tag.

Change-Id: I2bdd8e8312f3eb556b6a4277794d3c5d25008a12

openstack/python-brick-cinderclient-ext,openstack/python-brick-cinderclient-ext