fetch2: export DBUS_SESSION_BUS_ADDRESS to support authentication agents

fetch2: export DBUS_SESSION_BUS_ADDRESS to support authentication agents

Some users may want to use authenticated SSH connections with credentials stored
in a keyring, such as gnome-keyring.  These typically need a DBus session bus
connection, so pass DBUS_SESSION_BUS_ADDRESS into the fetcher environment.

To avoid the user needing to set it in their local.conf (which wouldn't be
usable) or adding it to the environment-cleansing whitelist (which would
potentially impact builds) allow the variables being passed to the fetchers to
come from the data store (first) or the original environment (second).

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>