Add continue option to grow command

Add continue option to grow command

To allow for reshape continuation '--continue' option is added
to grow command.
Function that will be executed in grow-continue case doesn't require
information about reshape geometry. All required information are read
from metadata.
For external metadata reshape can be run for monitored array/container
only. In case when array/container is not monitored run mdmon for it.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>