google_sql_user: Fixed import where host is an IPv4 CIDR

[modular-magician]

Previously it was not possible to import CloudSQL users for MySQL with host set to an IPv4 CIDR. The parsing of ID failed if host was a CIDR since it expected it to be on the form {project}/{instance}/{host}/{name} and an error was returned if there where more than three slashes in the ID.

With this fix if there are four slashes in the ID the third is considered part of the host argument.

sql: fixed importing `google_sql_user` where `host` is an IPv4 CIDR

Acceptance test run:

$ make testacc TEST=./google/services/sql TESTARGS='-run=TestAccSqlUser_mysql' 
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google/services/sql -v -run=TestAccSqlUser_mysql -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccSqlUser_mysql
=== PAUSE TestAccSqlUser_mysql
=== RUN   TestAccSqlUser_mysqlPasswordPolicy
=== PAUSE TestAccSqlUser_mysqlPasswordPolicy
=== CONT  TestAccSqlUser_mysql
=== CONT  TestAccSqlUser_mysqlPasswordPolicy
--- PASS: TestAccSqlUser_mysql (770.89s)
--- PASS: TestAccSqlUser_mysqlPasswordPolicy (950.89s)
PASS
ok      github.com/hashicorp/terraform-provider-google/google/services/sql      952.219s

Previously a import would fail with the message shown bellow. After the change this import succeeds with host set to 10.0.0.0/24.

$ terraform import google_sql_user.admin my-project/my-instance/10.0.0.0/24/admin 
[...]
        Error: Invalid specifier. Expecting {project}/{instance}/{name} for postgres instance and {project}/{instance}/{host}/{name} for MySQL instance

Derived from GoogleCloudPlatform/magic-modules#11523