consul_prepared_query do not read properly their failover, template and dns attributes

[remilapeyre]

resourceConsulPreparedQueryRead does not set properly the failover, template and dns attributes, this makes consul_prepared_query ignore external changes of those attributes.

This can be easily seen by running TF_SCHEMA_PANIC_ON_ERROR=1 make testacc TESTARGS="-count=1 -run=TestAccConsulPreparedQuery_basic":

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -count=1 -run=TestAccConsulPreparedQuery_basic -timeout 120m
?       github.com/terraform-providers/terraform-provider-consul        [no test files]
=== RUN   TestAccConsulPreparedQuery_basic
panic: failover.0.nearest_n: can only set full list