Provide a method to override cisco package via env

Cisco packages are the default for COI.  It is still possible
to use other sources, but doing so requires commenting out
one line in install.sh.  This change allows users to achieve
the same end without modifying the script: they can simply
set "vendor=stackforge" in their shell environment before
running the install script.

I'm open to sending the stackforge.install.sh stub upstream if anyone thinks it would be useful there, but it didn't seem to be.