Unhardcode and harden server key bits

Change-Id: I2d50330ec4d8bd37b36e75ede2801cfbdb140d6c
diff --git a/openssh/files/sshd_config b/openssh/files/sshd_config
index 9e5ee53..59da0a5 100755
--- a/openssh/files/sshd_config
+++ b/openssh/files/sshd_config
@@ -27,7 +27,7 @@
 
 # Lifetime and size of ephemeral version 1 server key
 KeyRegenerationInterval 3600
-ServerKeyBits 768
+ServerKeyBits {{ server.get('server_key_bits', '2048') }}
 
 # Logging
 SyslogFacility AUTH