Add lf-pip-install macro to handle pypi packages

This creates a generically reusable pip-install macro that can be used
to install pypi packages into a virtualenv. The script uses the first
package name from {pip-packages} as the name of the virtualenv it
creates.

Change-Id: Iabe4005efcb0a79af48d3c83d854f4e101414737
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>