Check for new versions of get-pip.py

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
diff --git a/.gitignore b/.gitignore
index 67ab722..c6900c8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,7 +14,7 @@
 files/*.qcow2
 files/images
 files/pip-*
-files/get-pip.py
+files/get-pip.py*
 local.conf
 local.sh
 localrc