Old configuration support is broken

[andriymoroz]

The commit "[dockers] Remove dependency to minigraph" breaks the support of old minigraphs which do not have neighbors listed in section <Devices>.
Option "-m" for sonic-cfggen helped to generate empty DEVICE_NEIGHBOR_METADATA. Without it it is not generated and buffers configuration template fails.
New minigraphs work ok.

[liatgrozovik]

@andriymoroz does it mean that we need as part of sonic upgrade to generate new minigraph?
@lguohan what should be the preferred behaviour to a user? If i remember correct we discussed that we will have minigraph as backwards compatibility, if this is not the case any more we need to provide different guidelines for the users.