twister: support testing multiple toolchain variants

Added integration_toolchains to allow building/testing with multiple
toolchains available in the environment.

This changes the output structure and adds another level in the path
under twister_out signifying the toolchain used. The toolchain used
(variant) is also part of the json output now.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>