Instructions update: install postfix separately, new privilege to mysql users, sockets dir. Fixes #3639

- As noted in #3596, I removed postfix from deps and added a note for the user to add it separately. 

- Updated update instructions and added a step to grant the additional privilege to the mysql user. Without it, backup [fails](https://groups.google.com/forum/?fromgroups=#!topic/gitlabhq/So4mYIRFsX8).

- Also, in a fresh installation the `tmp/sockets` directory must be created otherwise gitlab service fails to start. See #3639.