Add keystonemiddleware to requirements

Keystone middleware has been split. Keystone ec2_token middleware
has been included in the new repository. Keystone will depend on
the keystonemiddleware package and import ec2_token instead of
providing duplicated code.

Change-Id: I10534c1ca9883b9f98c998a6cb61e500f6c6ff2d
bp: split-middleware