Add support for different execution environment, e.g. sandbox

[IvanGoncharov]

Many APIs have multiple environments for execution of the same calls.
Examples are sandbox, geographical locations(to minimise latency), different instances of the same service, etc.

Currently, it impossible to describe multiple environments in the same document but @fehguy proposal from #640 to add array of hosts to spec:

 hosts:
   - scheme: http
     host: foo.com
     port: 8080
     basePath: /bar
   - scheme: https
     host: foo.com
     port: 8443
     basePath: /baz

It seems like good solution for this problem, but it missing few essential details:

• title field
• description field
• some mechanism to mark default host

[webron]

I assume both title and description would be optional.

[IvanGoncharov]

@webron Yes, definitely. It would be really strange to produce different titles for HTTP and HTTPS hosts.

[IvanGoncharov]

@webron I also think if you introduce support for multiple hosts you need to support marking default.
Previously it was choose between HTTP and HTTPS so it was done by tools automatically.
Simplest possible solution would be to add default property to elements of hosts.
I see only one problem with it: JSON Schema can't validate that it set to true for a single host.

I also think one host SHOULD be marked as default.

[webron]

JSON Schema constraints shouldn't dictate the spec. JSON Schema cannot and will not be able to fully validate the spec anyways, so it's a non-issue.