Support setting max network config file to load.

[Random-Liu]

For containerd/cri#889

For Kubernetes, only one default pod network is needed. This PR:

1. Changed WithDefaultConf to only load the default config.
2. Added WithAllConf to load all configurations.

We can continue using WithDefaultConf for Kubernetes, and for people who need multiple network support, they can use WithAllConf.
@abhi @crosbymichael

Signed-off-by: Lantao Liu lantaol@google.com

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.22%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   41.41%   41.63%   +0.22%     
==========================================
  Files           8        8              
  Lines         297      305       +8     
==========================================
+ Hits          123      127       +4     
- Misses        145      148       +3     
- Partials       29       30       +1

Impacted Files	Coverage Δ
opts.go	18.42% <54.54%> (+2.38%)	⬆️

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[crosbymichael]

LGTM