support other security protocols

The current cookbook only supports TLS for the email security.

This pull request deprecates node[:monit][:mail][:tls] in favor of a new attribute: node[:monit][:mail][:security].
node[:monit][:mail][:security] is a string.

Warnings are logged when:
- node[:monit][:mail][:tls]  is used.
- and/or both node[:monit][:mail][:tls]  and node[:monit][:mail][:security] are set.

node[:monit][:mail][:security] has precdence over node[:monit][:mail][:tls].