We force installs via pip, we should declare it

pbr contains code that very explicitly uses pip, which means that pip is
a requirement.

Note to packagers: it is possible to skip uses of pip, meaning that you
can totally use pbr-based things without needing to have them pip
install things.

Change-Id: Ifa472f344489295a5b8a1910bff2672087653547
Merge "We force installs via pip, we should declare it"