change /ds-routing/yes-no-gateway to POST request; lowercase JSON keys 

#### What's this PR do?
1) Changed /ds-routing/yes-no-gateway to a POST request since GET requests have a limited number of req.query params

2) Lowercased JSON keys ("PROVE", "DO", "KNOW", "PLAN") in campaign-start-config.json, changed necessary references in MCRouting.js

#### Jira Issue numbers:
SMS-48, SMS-50, SMS-47
#### How should this be manually tested?
Manually tested change in ds-routing with postman. POST request to '/ds-routing/yes-no-gateway' with params: 
phone: [phone number]
args: yes yes 
mdata_id: 9521
opt_in_path_id: 166253 

There should be a successful request to mcommons, with an acknowledgment returned in the terminal. 

Also manually tested change in /ds/handle-start-campaign-response with postman, following params: 

phone: [phone number]
args: prove (or 'do', 'know', 'plan')
mdata_id: 9521
opt_in_path_id: 166253