Updating LB enum to allow switch in default (#2232)

Switching the default zero value to UNSPECIFIED. This allows us
to better detect when the user has set a value for the enum. If
the user previously specified ROUND_ROBIN, it will still be
handled properly, but will be assigned a new enum value.

In addition, the value LEAST_CONN is now deprecated in favor of
a new value LEAST_REQUEST. It was previously poorly named and
only remains for backward compatibility.

There is a chance that this change could break existing tools
if they rely on binary marshaling/unmarshaling of the proto.
There are currently no tools within Istio that do this,
however.

Author: nmittler