XOP-369: Make RRDD plugins' data sources default=false

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
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>
CA-108508: Add a script to re-enable the RRDD plugin data sources

Script is 'xe-enable-all-plugin-metrics', which takes a single
argument 'true' or 'false'. This script will restart the toolstack.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #1298 from jonludlam/CA-108508

CA-108508