Refactor thrift for ext-data-source to generate only necessary structs

ext-data-source only needs a small subset of the thrift structures, so this
separates the dependencies between files so that just the necessary structs
are generated for ext-data-source. Afterwards, we can remove extra maven
dependencies which were using environment variables to get versions. While the
environment variables work when building the pom, they are not propagated to
dependencies so building fe/pom.xml ended up producing lots of warnings which
are now gone.

Change-Id: I267fe7bc7a54c3c21aad8c1ffce07cf1a1e07c5e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3748
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins