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: I1281de6e297fdae73ea285b0c7ccc4db3d64123d
Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

tox.ini

@@ -14,7 +14,7 @@ setenv = VIRTUAL_ENV={envdir}
          JUJU_REPOSITORY={envdir}/tmp/build
 passenv = http_proxy https_proxy
 install_command =
-  pip install {opts} {packages}
+  pip install --no-cache-dir {opts} {packages}
 deps =
   -r{toxinidir}/requirements.txt
 whitelist_externals = /bin/true /bin/echo /bin/mkdir /bin/ln