Make keyserver configurable and fix faraday (berkshelf version bump)

[dennisvink]

Allow for the default keyserver.ubuntu.com keyserver override.

[dennisvink]

The new faraday gem is incompatible with Berkshelf 3.x. I've bumped the version to ~> 4.0.0 to resolve this.

[sunggun-yu]

Thank you for the PR!

[sunggun-yu]

Hi, @megamoth !

Thank you for your PR. and, I have question for this PR.
I'm just curios, Is there any specific reason that make it configurable?

[dennisvink]

Greetings @sunggun-yu !

I've made it configurable to be able to continue with deploys when the default keyserver is down. It's not an uncommon occurance :-P

[sunggun-yu]

Thank you for your explanations.
I'll merge it. however, I'll change some portion of it. I've decided to provide it as ['mongodb3']['package']['repo'] attribute in order to make it configurable. it will be worth to resolve #10 like issues.

[sunggun-yu]

opps, I confused, I'm not saying about keyserver. I'm saying about uri. sorry for make you confused.