create flow function to build OTG flow config

[bortok]

Example usage:

otgen create tcp -s 1.1.1.1 -d 2.2.2.2 -p 80 --rate 1000pps

should produce an OTG config on stdout that can be further passed to otgen run stdin

TODO:

• flow boilerplate with TCP
• dst parameter - single value
• src parameter - single value
• smac/dmac params - single value
• rate parameter
• count parameter - fixed_packets
• size parameter - fixed
• port parameter
• flow name
• metrics enable
  • latency modes
• tcp | udp selector
• ipv4 | ipv6 selector
• icmp proto
• review env vars for envsubst
• mac address env vars
• ip address env vars
• select tx and rx ports
• add alternative to create to read config from stdin and add to it
• swap

Moved these to #19

• delay parameter for flows
• make flow a default subcommand
• add array of values
• add increment and decrement values