Changes to support xDS Federation

[arvindbr8]

as per a47 xDS Federation spec we need to add the authorities field.
For the authorities field, we want two entries:

• Empty string: This will be used for all new style resource names which contain no authority
• "trafficdirector.googleapis.com:443": This is for new style resource names which explicitly specify TD
  For both these entries, we will leave the value empty or {}. That way, they will still both end up talking to TD with the server config specified by the top-level

Note that in authorities map authority name's value is empty. This is to force default to the xds_server_uri specified in the top level xds_servers field.

Fixes task 1 in #32

[easwars]

@ejona86 : I have reviewed this PR and looks fine to me. Do you mind taking a quick look as well? Thanks.

[ejona86]

The changes look fine, but this doesn't actually fix #32 entirely, correct, because we want the c2p stuff to be present?

[easwars]

The changes look fine, but this doesn't actually fix #32 entirely, correct, because we want the c2p stuff to be present?

Yes, this change only addresses basic federation. The c2p changes would follow after this.

[arvindbr8]

Yes I can add c2p changes as a separate task in #32