allow for soft updating of global-requirements

This creates a devstack REQUIREMENTS_MODE which is how we handle
syncing of global requirements. The default is 'strict', which is
current behavior. There is a new 'soft' mode which does a
--soft-update for projects *not* found in projects.txt, which lets
them specify additional requirements.

Change-Id: I4aa606514131b5dde67d87f5c8db5a3f3e50fc03
Depends-On: I1f195ef9ff1509659848e14ec9936ff6f66a6496
Use updated trove mysql image location

Use the updated trove mysql image at:
tarballs.openstack.org/trove/images/ubuntu/mysql.qcow2
instead of the older deprecated image at:
tarballs.openstack.org/trove/images/ubuntu_mysql.qcow2/ubuntu_mysql.qcow2

Change-Id: If76f25dfe0f975faf1883f0a6d993c21b26e1b29
Merge "Use updated trove mysql image location"
Merge "allow for soft updating of global-requirements"
Merge branch 'master' of https://github.com/openstack-dev/devstack