Run pip again if it fails

Often, pip can fail with "connection reset by peer".
Run it again keeping cache of already downloaded packages.

Always use an empty build dir for pip (it can forget to
save archives if the package resides in build dir).

Show progress for every file built by py2rpm.

Change-Id: I122670d05eba1ca93f209e1e22d45a5f8c0598b6