DBT-747: Removing the describe table statement each tables rather use the show queries to gather metadata

[vamshikolanu]

Describe your changes

We have identified a bug where dbt-impala is trying to scan all schemas mentioned in dbt-project.yml and run a describe table on all tables of those schemas to figure out whether it's a view or a table that is causing large delays in the execution.

The problem was that Impala didn't implement the show views command which was fixed as part of https://issues.apache.org/jira/browse/IMPALA-3268.

Inorder to start using this adapter going further after this fix, we expect users to include this patch: https://issues.apache.org/jira/browse/IMPALA-3268

Internal Jira ticket number or external issue link

https://jira.cloudera.com/browse/DBT-747

Testing procedure/screenshots(if appropriate):

https://gist.github.com/vamshikolanu/977f79dc14acf16d512058410074718f

Checklist before requesting a review

• I have performed a self-review of my code
• I have formatted my added/modified code to follow pep-8 standards
• I have checked suggestions from python linter to make sure code is of good quality.