The 'pypi' mirror element is generating invalid pip.conf files
when more than one "DIB_PYPI_MIRROR_URL" is specified.
This patch fixes the pip.conf file rendering to use a proper form
when there are multiple "extra_index_url" provided.
Closes-Bug: #1839558
Change-Id: Ibda0e7390955683560e09b486f636775643ff57c