IPv6 support

[horiagunica]

Add IPv6 support to modules and add one example to demonstrate usage in IPv6 environment. This should include the following:

• [x] #17
• [x] #18
• [x] #19
• [x] #20
• [x] #38
• [x] #40
• [x] #21

[pavelrn]

• Support for dual-stack subnets (External IPv6 addresses) added into modules/vpc
• Support for dual-stack External L7 LB added into modules/lb_http_ext_global. The LB terminates IPv6 and uses IPv4 to communicate to the backend

The main issue body is updated to reflect the changes required