Increase diego-ssh RSA host key from 2048 to 4096 bits

Strengthen the SSH host key used by diego-sshd in LRPs by changing the
default RSA key size from 2048 to 4096 bits.

Author: plamen-bardarov

lib/cloud_controller/diego/ssh_key.rb

@@ -8,7 +8,7 @@ module VCAP
   module CloudController
     module Diego
       class SSHKey
-        def initialize(bits=2048)
+        def initialize(bits=4096)
           @bits = bits
         end