Re-inserted a rm statement for firstboot file

Issues:
Fixes F5Networks#75

Problem:
* The firstboot is given to the BIG-IP from onboarding
* After the initial, firstboot init cycle, this file should be removed
  * Without it's removal config collisions could occur
  * Without it's removal reboot cycles take longer

Analysis:
* This inserts the removal statement after all first boot inits

Tests:
None.  This is a startup scripting for onboarding.  While we have some
tests in regards to this, it would be very difficult to tell whether or
not the first boot init was done twice after a reboot.

lib/f5_image_prep/startup

@@ -247,6 +247,11 @@ function main() {
 		log "Cannot run OpenStack auto-configuration on non-VE platforms, quitting..."
     fi
 
+    # Delete the /config/firstboot file, we have completed initial setup
+    if [[ ${IS_FIRST_BOOT} == "true" ]]; then
+        rm -f $FIRST_BOOT_FILE
+    fi
+
     wait_condition_notify
 
     cleanup_user_data