Update ubuntu - remove 17.10 & add 18.10

src/ubuntu.ipxe

@@ -10,8 +10,8 @@ set os Ubuntu
 clear ubuntu_version
 menu ${os} - ${arch_a} - Image Sig Checks: [${img_sigs_enabled}]
 item --gap Latest Releases
+item cosmic ${space} ${os} 18.10 Cosmic Cuttlefish
 item bionic ${space} ${os} 18.04 LTS Bionic Beaver
-item artful ${space} ${os} 17.10 Artful Aardvark
 item xenial ${space} ${os} 16.04 LTS Xenial Xerus
 item trusty ${space} ${os} 14.04 LTS Trusty Tahr
 item --gap Older Releases