placement can throw exception if webob<1.8 in use

[Sylvain Bauza]

With https://bugs.launchpad.net/nova/+bug/1773225 we addressed a better way to lookup HTTP headers but unfortunately the WebOb change wasn't backwards compatible with webob versions older than 1.8.

Since upstream requirements still support WebOb in 1.7.1 and higher (up to 1.8.2) [1], some Placement deployments could still using this 1.7 version and throw an AttributeError exception. For example, TripleO CI is reporting an incompatible usage in [2]

[1] https://github.com/openstack/requirements/blob/6766f353b2e814ecbd36dd08216022cf2024c03b/lower-constraints.txt#L18

[2] https://bugzilla.redhat.com/show_bug.cgi?id=1596308

[Chris Dent (cdent)]

This was fixed by raising the lower constraint: https://review.openstack.org/#/c/579110/