CA-135187: Filter out restricted device_config keys

A commit fixing SCTX-1616 now causes PBD create to throw an error if given
a device_config with the SRmaster key in (internal use only).

However, if this happened to exist from a time when this was not forbidden then
pool join will fail since the slave uses the device_config from the master's
PBD record which might contain this key.

This commit filters this key out in the pool join code-path.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>