Add SLES mock to ext/build_defaults.yaml

We now build for SLES inside a mock chroot using the `mock` tool, which means
we have to specify the mock config to use. The mock configs for sles have been
propagated by the rpm-builder module, and are ready to go. This commit adds the
mock for SLES 11.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>