move cache dir from runtimeConfig to CNIConfig

[mars1024]

Yes, I also agree on that runtime should not change cache dir freely.
So maybe there is another way, we can put cache dir from runtimeConfig to CNIConfig, and add a new constructor func NewCNIConfig(path []string, exec invoke.Exec, cacheDir string) *CNIConfig, if we use the old constructor NewCNIConfig(path []string, exec invoke.Exec) *CNIConfig, the cache dir will be a default value. Once cache dir is set in CNIConfig, it won't be changed by any CNI user.

Originally posted by @mars1024 in #678

[mars1024]

close via #682