Make tenant_network_mask_bits default setting consistent

tenant_network_mask_bits in the sample config is 29, but
in the config.py it is 28.

According to the bug report the 28 is the desired value.

Fixing bug 1105199

Change-Id: I45e73dc436bd6aea109c1be934bc7de2d950ef2b
Merge "Make tenant_network_mask_bits default setting consistent"