Add falcon to global-requirements

Falcon is the default wsgi framework used in Marconi. Since Marconi was
accepted as an incubated project, this patch adds this requirement to
global-requirements as part of Marconi's integration with OpenStack's
infrastructure and tools.

Change-Id: I179c875e0fc8adfad2323b31a063d2f5b7d52623
Merge "Add falcon to global-requirements"