Add Juniper to the vendors listed in fixtures.php

This just adds Juniper to the list of vendors in the fixtures file. I presume there is no specific required syntax for the "Short Name" and "Nagios Name"? (I just added something analogous to the other vendor-entries). I realise there may be the desire not to add/acknowledge the vendor yet in the "preset values" (if the support isn't robust enough yet, etc). In that case I thought perhaps this PR can just sit in waiting so no-one forgets to add to the fixtures file when the time is right).