Modify role to work in RHEL based server (plus some improvements)

[badnetmask]

Thanks for this role. I found it while looking into how to implement the Digital Ocean tutorial. Very handy.

I made some changes to make it work in RHEL based servers (only tested in RHEL7 so far):

• Replaced "apt" tasks with "package".
• Modified some Postfix options which are not compatible with 2.x (still in use in RHEL7).
• Fixed some file permissions which Postfix was not happy about.

Plus I also added some improvements:

• Added a note in README about pre-configuring MySQL, and a required Python module.
• New parameters to configure Postfix as a relay host and SMTP (sending) security level.
• Optional task to add example Dovecot IMAP users.

I hope this is of your interest.

[thatarchguy]

This is awesome, thank you for helping improve it.