http.c: Add ability to create multiple HTTP servers

Previously, it was only possible to have one HTTP server in Asterisk.
With this patch it is now possible to have multiple HTTP servers
listening on different addresses.

Note, this behavior has only been made available through an API call
from within the TEST_FRAMEWORK. Specifically, this feature has been
added in order to allow unit test to create/start and stop servers,
if one has not been enabled through configuration.

Change-Id: Ic5fb5f11e62c019a1c51310f4667b32a4dae52f5