Don't pass swift headers as None

If we set an header as None for a container swiftclient would just pass
it to httplib which convert it to string and set the None string into
it. This could not be changed in swiftclient to keep backward
compatibility lords happy so make sure that heat doesn't do that.

Closes-Bug: 1259571
Change-Id: Ida1d2cec87e44264f9a7d69a1e220b544b33b63c
Merge "Don't pass swift headers as None"