include description field for user tokens

[drewmullen]

closes: #83

Description is not supported by Org or Team tokens. Only implemented for user tokens

CGO_ENABLED=0 VAULT_TOKEN= VAULT_ACC=1 go test -v -tags='vault-plugin-secrets-terraform' $(go list ./... | grep -v /vendor/ | grep -v /integ) --run=TestUserRole -count=1 -timeout=10m -parallel=4
?   	github.com/hashicorp/vault-plugin-secrets-terraform/cmd/vault-plugin-secrets-terraform	[no test files]
=== RUN   TestUserRole
=== RUN   TestUserRole/Create_User_Role_-_fail
=== RUN   TestUserRole/Create_User_Role_-_pass
=== RUN   TestUserRole/Read_User_Role
=== RUN   TestUserRole/Update_User_Role
=== RUN   TestUserRole/Re-read_User_Role
--- PASS: TestUserRole (0.00s)
    --- PASS: TestUserRole/Create_User_Role_-_fail (0.00s)
    --- PASS: TestUserRole/Create_User_Role_-_pass (0.00s)
    --- PASS: TestUserRole/Read_User_Role (0.00s)
    --- PASS: TestUserRole/Update_User_Role (0.00s)
    --- PASS: TestUserRole/Re-read_User_Role (0.00s)
PASS
ok  	github.com/hashicorp/vault-plugin-secrets-terraform	0.264s

running tests locally:

$ vault write sys/plugins/catalog/terraform sha256="414b412a4f024195d4585c1942be9ea8354c281eaeb0ed72ab318477a9d80793" command="vault-plugin-secrets-terraform"
$ vault secrets enable -plugin-name='terraform' plugin
$ vault write terraform/config token=$TFE_TOKEN
$ vault write terraform/role/tfc-mgmt user_id=user-<redacted> ttl=7200 description="test description"
$ vault read terraform/role/tfc-mgmt
Key            Value
---            -----
description    test description
max_ttl        0s
name           tfc-mgmt
ttl            2h
user_id        user-<redacted>