Use 'sudo -H' when installing pip things

New versions of pip creates and uses $HOME/.cache.  diskimage-builder also
uses this path, but it now gets created by root when using pip, causing
later image builds to fail on permissions issues.  This updates sudo usage
during pip installations to pass -H, ensuring pip uses /root/.cache instead
of the stack user.

Similar fixes were requried to devstack, see review #143501.

Change-Id: I8c3c460e9dac1ed1463df37de5810d4faf8512ad