Http Server: List all envinronment variable

This adds an API that returns a list of all the environment variable.

This patchs also adds a test to the testenv.py that verify that the
variable that was added in the test is part of the the list return from
the new API.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>