DBT-688: Fix default value of incremental_strategy.

[vamshikolanu]

Describe your changes

config.get() default value is not working as expected. It's returning None instead of taking the default value as append. Hence explicitly setting the default incremental_strategy value to append

Internal Jira ticket number or external issue link

https://jira.cloudera.com/browse/DBT-688

Testing procedure/screenshots(if appropriate):

basic tests: https://gist.github.com/vamshikolanu/be58a8b74b7fd2fac02e20301e892722
insert overwrite test (verbose): https://gist.github.com/vamshikolanu/7bd533a6178f836501dfb61d36ba15d5
append test (verbose): https://gist.github.com/vamshikolanu/f2ebf2a4d4a309417ae465d0ce65b76a

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.

[niteshy]

LGTM