Allow loading datacenters from a custom location

[viktors]

This is to allow updating datacenters (stored in a different location) without having to update the gem. This PR has two changes:

• allow specifying custom datacenter path in load! method
• load datacenters on the first request instead of loading it when loading the gem

[olleolleolle]

@frewsxcv Is there a way to get this change merged? Like, say, splitting the changes in two separate Pull Requests?

(I'm just asking as a triage question - this is the final open PR.)