Migrate to oslo.context

That's just a matter of adding requirements.txt entry, fixing imports,
and dropping the corresponding oslo-incubator module from the tree.

While at it, made all imports to import the module into 'oslo_context' and
not just 'context', so that we don't override the module locally in
multiple methods that receive their context arguments with the same
name, making the library inaccessible from inside those methods.

Change-Id: Ie62af970b3b7f225de453e56c01abc4b12af8f5e
Closes-Bug: #1401054
Merge "Migrate to oslo.context"