Minor tweaks

- Linked "Software Defined Security" to https://securosis.com/blog/new-whitepaper-a-practical-example-of-software-defined-security

- First mention of DevOps is not explained. Added note that it will be explained in the next section

- Tweaked wording on patching. I refuse to believe that immutable servers will completely replace patching. Operationally, that's not entirely practical.

- Changed 'only' to 'roughly'...my code, for example, takes more lines of code than most to work properly :)