Add "depth" as a valid SSL option

[tianon]

Closes #144

[tianon]

This should be usable via RABBITMQ_SSL_DEPTH=2, as an example.

[tianon]

Here's an example generated configuration: (obviously using a bogus cert file, because I just wanted to test the config generation 😇)

[ { rabbit, [
	{ loopback_users, [ ] },
	{ tcp_listeners, [ ] },
	{ ssl_listeners, [ 5671 ] },
	{ ssl_options, [
		{ cacertfile, "/etc/passwd" },
		{ certfile, "/etc/passwd" },
		{ depth, 2 },
		{ fail_if_no_peer_cert, true },
		{ keyfile, "/etc/passwd" },
		{ verify, verify_peer }
	] },
	{ hipe_compile, false }
] } ].

[michaelklishin]

👍