Add DNS provider for RackCorp

[ldez]

Closes #3101

Ping @jstangroome, can you run the command with your domain, credentials, etc. (and put the output of this command to a comment)?

make build
rm -rf .lego

RACKCORP_API_UUID="xxx" \
RACKCORP_API_SECRET="yyy" \
./dist/lego run --dns rackcorp -d '*.example.com' -d example.com -s letsencrypt-staging

Note that the wildcard domain is important.

How to test this PR?

1. You need Go
2. Check out the PR:

   git clone https://github.com/ldez/lego.git
   cd lego
   git checkout feat/dns/rackcorp

3. Compile lego:
   • if you have make: make build
   • if you don't have make: go build -o dist/lego ./cmd/lego
4. Run the following command with your information (email, domain, credentials):

   RACKCORP_API_UUID="xxx" \
   RACKCORP_API_SECRET="yyy" \
   ./dist/lego run --dns rackcorp -d '*.example.com' -d example.com -s letsencrypt-staging

   The wildcard domain is important
5. Before each run of the command, you should clean your local environment:

   rm -rf .lego