Issue #77 - Fix changelogdb param issue
    
    Bug description: self.dbdir used before an assignment
    in Changelog(conn) __init__ part, while self.dbdir is defined
    in __initPart2 of DirSrv object.
    
    Fix description: Add self.changelogdb to __initPart2 with default value.
    Assign a new value in conn.changelog.create().
    
    https://pagure.io/lib389/issue/77
    
    Reviewed by: mreynolds (Thanks!)