Update MaxStartups to 10:30:60 per CIS 5.1.18 benchmark recommendation

- Change MaxStartups from default 10:30:100 to 10:30:60 in sshd_config
- Update corresponding rspec test assertion

Author: karthickaliappa

bosh-stemcell/spec/support/os_image_shared_examples.rb

@@ -238,8 +238,8 @@
       expect(sshd_config.content).to match(/^MaxAuthTries 3$/)
     end
 
-    it "sets MaxStartups to 10:30:100" do
-      expect(sshd_config.content).to match(/^MaxStartups 10:30:100$/)
+    it "sets MaxStartups to 10:30:60" do
+      expect(sshd_config.content).to match(/^MaxStartups 10:30:60$/)
     end
 
     it "sets PermitEmptyPasswords to no (stig: V-38614)" do

stemcell_builder/stages/base_ssh/apply.sh

@@ -25,7 +25,7 @@ sed "/^ *MaxAuthTries/d" -i $chroot/etc/ssh/sshd_config
 echo 'MaxAuthTries 3' >> $chroot/etc/ssh/sshd_config
 
 sed "/^ *MaxStartups/d" -i $chroot/etc/ssh/sshd_config
-echo 'MaxStartups 10:30:100' >> $chroot/etc/ssh/sshd_config
+echo 'MaxStartups 10:30:60' >> $chroot/etc/ssh/sshd_config
 
 sed "/^ *PermitEmptyPasswords/d" -i $chroot/etc/ssh/sshd_config
 echo 'PermitEmptyPasswords no' >> $chroot/etc/ssh/sshd_config