Use roles from the network request

[oguzkocer]

For updating the users in people management, we should use the /v1.1/sites/$site/roles to get a possible list of roles instead of using an array resource to be able to support Jetpack sites.

[maxme]

@oguzkocer should we move this one to the FluxC repo? (and implement it in the Site Store?)

[oguzkocer]

We should @maxme, I just added an issue for it.

[oguzkocer]

Re-opening this issue since we need to make a lot of changes in the main repo to support it.