People can leave their devstack installs around for a long time, and
in the mean time new versions of pip can be released.
The current check does not download a new version if an old one
exists. We want to check for new versions, but we also don't want the
gate jobs trying this sometimes unreliable fetch.
So add a flag-file that tells devstack if it downloaded get-pip.py
originally. If so, on each run check for a new version using curl's
"-z" flag to request only files modified since the file's timestamp.
Change-Id: I91734528f02deafabf3d18d968c3abd749751199
Closes-Bug: #1429943
7c4ce9edba