XOP-369: Add a config parameter to set the 'default' for plugins' DSs

data sources provided by plugins currently are assumed to be
default=false. To change this, add a line to the config file, which
is found at /etc/xcp-rrdd.conf:

'plugin-default = true'

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>