cmd/flow: validate nodes-count before generation

[immanuwell]

fake flow blows up on --nodes-count 0 with a panic.

This validates --nodes-count before generation, and adds a test. Small fix, but it saves a kinda dumb crash.

Repro:

1. cd cmd
2. go run . flow --nodes-count 0

Before:
panic: invalid argument to IntN

After:
--nodes-count must be at least 1

[kaworu]

Thanks!

[kaworu]

@immanuwell can you please fix or add an ignore tag to the golangci issue please

  Error: /home/runner/work/fake/fake/cmd/internal/cmd/flow/flow_test.go:29:22: string `10.0.0.0/8` has 4 occurrences, make it a constant (goconst)
  			opts.sourceCIDR = "10.0.0.0/8"
  			                  ^
  1 issues:
  * goconst: 1

[kaworu]

goconst