Add option to enable tlsproxy

[envy]

Add an env var to optionally enable the tlsproxy in master.cf

This fixes #222.

[bokysan]

Hello.

Thank you for the commit and I really do appreciate the work done. However, as it stands now, I cannot accept it:

1. Setting ENABLE_TLSPROXY to false will not disable an already enabled TLS proxy.
2. There are no tests associated with the provided commit.
3. The feature will not work if somebody changed the master.cf by hand and removed the line #tlsproxy.... Even if the line is just changed to # tlsproxy... this commit will fail.
4. Variable should have a POSTFIX_ prefix, at least, to prevent polluting the global namespace.
5. If we're implementing functionality to change master.cf we should enable other features as well, not just and merely tlsproxy. So the commit should be more generic.

[envy]

Sorry, I don't have the time to implement all that.