optimize size and time using "--no-cache-dir"

Using "--no-cache-dir" flag in pip install ,make sure dowloaded packages
by pip don't cached on system . This is a best practise which make sure
to fetch ftom repo instead of using local cached one . Further , in case
of Docker Containers , by restricing caching , we can reduce image size.
In term of stats , it depends upon the number of python packages
multiplied by their respective size . e.g for heavy packages with a lot
of dependencies it reduce a lot by don't caching pip packages.

Further , more detail information can be found at

https://medium.com/sciforce/strategies-of-docker-images-optimization-2ca9cc5719b6

Change-Id: Ib152ccd75af3b5987ac9729033e4be88cfb88f89
Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

README.rst

@@ -270,7 +270,7 @@ Dev install
 The following playbook will produce a modified image with Python source
 code installed via pip. To minimize dependencies within the container
 we generate the sdist locally and then copy it into the resulting
-container image as an sdist tarball to run pip install locally.
+container image as an sdist tarball to run pip install --no-cache-dir locally.
 
 It can be used to pull a review from OpenDev Gerrit:
 

templates/Dockerfile-dev.j2

@@ -9,7 +9,7 @@ RUN /bin/bash -c 'PKG="$(command -v dnf || command -v yum)"; \
     if [ $PKG_MGR == "dnf" ]; then $PKG install -y python3-pip; PIP=pip3; else $PKG install -y python-pip; \
     PIP=pip; fi; \
     cd /; \
-    for X in $(ls /root/refspec_projects/*.tar.gz); do $PIP install $X; done; \
+    for X in $(ls /root/refspec_projects/*.tar.gz); do $pip install --no-cache-dir $X; done; \
     rm -Rf /root/refspec_projects'
 
 USER "{{ original_user }}"

tox.ini

@@ -5,7 +5,7 @@ skipdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install --no-cache-dir -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 passenv =
     CURL_CA_BUNDLE