Change grant provider to ignore grants for non existing users.

In the grant provider users are fetched by querying mysql.user table. Grants
for those users are fetched using show grants for... syntax. This can lead to
errors, when some of the users in mysql.user table do not have currently
active grants.

This happens at least when MySQL is started with --skip-name-resolve option,
and there are users with the hostname part specified as a FQDN. Such users are
created by mysql_install_db. This leads to problems if mysql::account_security
is included for the node and skip-name-resolve is specified in override_options
hash for mysql::server.