Dependencies V2 not friendly for new users

[black-adder]

Requirement - what kind of business use case are you trying to solve?

Make dependencies v2 more new user friendly.

Problem - what in Jaeger blocks you from solving the requirement?

#1328 added a new schema mapping for dependencies_v2 with is by default used by cassandra-create-schema scripts, however the v2 table is not by default used by jaeger (you have to pass in a flag). This means that new users will not be able to query dependencies by default.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Do a DESC query on the cassandra jaeger keyspace on startup and see if the dependencies_v2 table is available. If so, use v2, else use v1.

Any open questions to address

How will this work with the flag? Should we just go ahead and deprecate it?