Environment Provider and Agent SPI

[garagatyi]

Overview
This epic introduces usage of multi-machine environment definition syntax.

Tasks

• Separate environment runtime management from workspace runtime management Separate environment runtime management from workspace runtime management #1864
• Change Workspace environment model Change Workspace environment model #1818
• Make Che with multi-machine environments backward compatible Make Che with multi-machine environments backward compatible #1820
• Machine Agent specification and implementation Machine Agent specification and implementation #1823

Impl Plan and Dependencies:
2 goes after 1
3 can be done after 2