galera: Allow empty password for "check_passwd" parameter

When using a custom "check_user" without a "check_passwd" *and* the
user has a .my.cnf file in its home directory, the monitoring fails
like:

Operation promote for galera:0 (ocf:heartbeat:galera) returned: 'unknown error' (1)
> stderr: INFO: Node <d52-54-77-77-01-02> is bootstrapping the cluster
> stderr: ERROR: MySQL is not running
> stderr: INFO: Waiting on node <d52-54-77-77-01-01> to report database status before Master instances can start.
> stderr: INFO: MySQL is not running
> stderr: INFO: MySQL is not running
> stderr: ERROR 1045 (28000): Access denied for user 'monitoring'@'localhost' (using password: YES)
> stderr: ocf-exit-reason:Unable to retrieve wsrep_cluster_status, verify check_user 'monitoring' has permissions to view status
> stderr: ocf-exit-reason:local node <d52-54-77-77-01-02> is started, but not in primary mode. Unknown state.
> stderr: ocf-exit-reason:Failed initial monitor action

This happens because the credentials from ~/.my.cnf are used. There is
now a new parameter called "check_passwd_use_empty" (defaults to 0 so
no behavior change) which can be used to use an empty password.