Added ability to use already hashed passwords in mysql::db class

Added boolean "hashed_password" to avoid hashing an already hashed $password.
This allows to not having plain passwords stored.

When "$hashed_password => true" $password should contain the same hash MySQL uses and show when executing "SHOW GRANTS .."

if $hashed_password is not present or set to false it hashes the given $password. This is the default behavior.

Usage:
    mysql::db { 'sbtest':
        user => 'database',
        hashed_password => true,
        password => '*0380BEA27363E56C37F0BFDA438F429080848051', # hash equals to sbtest
        host => 'localhost',
        grant => 'ALL',
    }