bind plugin should create slave subdirectory
The bind plugin should check for and create the slave subdirectory.
The bind plugin generates the directory name to use for slave zones by appending "/slave" to the end of the zonefiles directory path, with no way to configure anything differently. When that path is set to a non-default location which does exist (eg. /etc/bind/ispconfig/) the bind plugin attempts to write zone files to the "slave" subdirectory without checking to see if it needs created first. If the path is generated and not configurable, it should also be created. An alternate solution would be to let the slave zonefile path be specified in the UI. Or better, do both.