[feature request] Add check to see if mail is installed/configured

[vinyar]

Preface:
Chef server requires mail for some of its core functionality.

Present solution:
Chef pre-req docs makes a note that some form of SMTP (postfix/sendmail/etc..) should be available on the box and configured by the user.

Proposed enhancement:
A pre-flight check can point out that mail is not configured. This simple step will save a lot of time for non-linux users in troubleshooting issues such as 'why my reset password is not working'. This is particularly useful in scenarios where people with zero linux experience that using Chef.

[vinyar]

similar bugs in analytics, which has heavier reliance on mail than chef-server: https://chefio.atlassian.net/browse/CORE-1