Currently, when the repo server stores files like OpenStack's
upper-constraints.txt, the cache settings cause pip to cache this
file. When updating this file or experimenting with different
constraints, this cache can cause unexpected results as pip may
deploy different versions to those currently specified in the file.
This change instructs pip or other clients not to cache responses,
and matches the OpenDev origin's response headers for requirements
files.
Similar issues could apply for wheels and similar if experimental
changes are made without adjusting version numbers.
Change-Id: I7fdb62f79590deb118ecac2aac71984bd32685f1
52aaea359a