Sceptre: Improve cross region/account support

[theseanything]

We have limited support for multi region or account deployments. Currently you can have separate "environments" in different regions or accounts - (scoped by your aws cli profile). However, there is no support for stacks within a single "environment" to be in different regions/accounts. The currently implementation also restricts the ability to reference stacks in other account or regions for api operation such as !stack_output.

Proposed implementation would be to add a class attribute ConnectionManager to store and keep track of different boto sessions associated with each profile or stack_name. This would allow us to reference sessions used by other stacks and interact with that particular account/region. Also adds stack level granularity to connection_manager object rather than sharing one at an "environment" level.

This addresses #89 and #85

[theseanything]

Currently under development in branch cross-account-region-support. Bulk of implementation done - having issues with the integration tests passing.

Dropping support for Sceptre role assumes (iam_role) in favour of using profile exclusively. Betting mapping of profiles between users could be implemented as a feature after this PR.

[JohnVonNeumann]

I like how everything that I think "yeah that would be useful", you are already working on.

[campanellaf]

Is this still ongoing?

[ngfgrant]

This is resolved in v2.