Add tools/install_pip.sh

Install a known working recent version of pip that handles installation
dependencies more correctly than before.  Extract to a separate script
so it can be used apart from stack.sh.

* Install distro setuptools if it not already present
* Install pip from source tarball as get-pip.py proved to be unreliable
* Remove python-distribute and python-pip from all prereq files,
  move python-setuptools to 'general'
* Remove the earlier unfubar_setuptppls() call that attenpted to fix this
* Only update requirements.txt when no changes in repo

Tested on Precise, F18 and CentOS6.
* Fedora and RHEL allow pip to install packages ON TOP OF RPM-installed
  packages.  THIS IS BROKEN.  And is one reason we have to be so picky
  about order and so forth.

Change-Id: Ibb4b42119dc2e51577c77bbbbffb110863e5324d
diff --git a/files/apts/general b/files/apts/general
index fdf8e20..fcf0b5b 100644
--- a/files/apts/general
+++ b/files/apts/general
@@ -1,6 +1,6 @@
 bridge-utils
 pylint
-python-pip
+python-setuptools
 screen
 unzip
 wget