Support version selection for database plugins

[tomhjp]

Supports specifying a plugin_version when configuring a database plugin.

To test locally:

make dev
go build -o plugin_directory/postgres-1.11.0-rc1 plugins/database/postgresql/postgresql-database-plugin/main.go
cp plugin_directory/postgres-1.11.0-rc1 plugin_directory/postgres-1.11.0-rc2

# Run postgres + vault
docker run --rm --name postgres -e POSTGRES_USER=root -e POSTGRES_PASSWORD=root -d -p 5432:5432 postgres
VAULT_DEV_ROOT_TOKEN_ID=root vault server -dev -dev-plugin-dir=pluin_directory &
export VAULT_ADDR=http://127.0.0.1:8200
export VAULT_TOKEN=root

# Register and configure plugins
SHA256="$(shasum -a 256 plugin_directory/postgres-1.11.0-rc1 | awk '{print $1}')"
vault write sys/plugins/catalog/database/pg sha256=$SHA256 command=postgres-1.11.0-rc1 version=v1.11.0-rc1
vault write sys/plugins/catalog/database/pg sha256=$SHA256 command=postgres-1.11.0-rc2 version=v1.11.0-rc2
vault secrets enable database

# Explicitly use rc1
vault write database/config/pg1 plugin_name=pg connection_url="postgresql://{{username}}:{{password}}@localhost:5432/?sslmode=disable" allowed_roles="*" username=root password=root plugin_version=1.11.0-rc1

# This will select and pin the latest versioned pg plugin registered, rc2
vault write database/config/pg2 plugin_name=pg connection_url="postgresql://{{username}}:{{password}}@localhost:5432/?sslmode=disable" allowed_roles="*" username=root password=root

# Prove the correct binaries are running
ps aux | grep postgres

[mpalmi]

Looks good to me. I haven't had the chance to take it for a spin yet, but that shouldn't hold up the merge. Nor should the nits in my review.

[mpalmi]

+1 this is a confusing overloading of terms. A bit of clarity is good.

[swenson]

So far, looks great.

[tomhjp]

Thanks for the reviews so far - I think it's ready for another pass @swenson

[swenson]

LGTM