+ added redis elasticache as a built-in plugin

[maxcoulombe]

The redis elasticache database plugin is a new secrets backend allowing vault to manage static roles for AWS ElastiCache clusters for Redis. See the repo here for full details.

This PR bundles the new plugin into the vault binary, as we do with most other database plugins.

Tested locally:

cd $PROJECT_FOLDER/vault
make dev GCFLAGS="all=-N -l"
./bin/vault server -dev -dev-root-token-id="root"

./bin/vault secrets enable database
./bin/vault write database/config/redis-db plugin_name="redis-elasticache-database-plugin" allowed_roles="*" url="${ELASTICACHE_URL}"
./bin/vault write database/static-roles/redis-role db_name="redis-db" username="vault-test" rotation_period=300s

[Zlaticanin]

Small nit for the changelog. Following the formatting, it should be something like this maybe:
**Redis Elasticache DB Plugin**: <text>
Otherwise it LGTM 👍