Delete docker configration after provision

This pull-request is for fixing the issue #72.

This pull-request makes vagrant-proxyconf do docker proxy config only once at the beginning of the provision. 

The old version does docker proxy config twice at beginning of a provision and after the provision. If vagrant-proxyconf does docker proxy config after a provision, docker can fail to restart the daemon. Docker can take effect with only a configuration at the beginning of the provision.

We have checked this pull-request working well on:
- [X] RHEL
- [X] Ubuntu
- [X] boot2docker by @dduportal, thank you.

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There are remaining problem(s).
-  [X] vagrant-proxyconf can not take effect if docker installation in provision phase.
  - I am going to close this phenomenon as a limitation.