add new __parse_disks() in ConfigurationDomain
remove useless method StoragePool.created()

The test is wrong: it returns False if the StoragePool was created in another
instance of Cloubed. To avoid bugs, StoragePool.create() should be reentrant.

The method is not used anywhere else in the software.
added new method StoragePool.get_status()
remove overwrite parameter of StoragePool.created()

It was always False so its usefulness was quite limited.
make method StoragePool.create() reentrant

This implies a new method find_storage_pool() which is also used by
get_status().
add new method StorageVolume.get_status()
add new method Network.get_status()
add new method Domain.get_status()
add new status action to cloubed script

This prints to screen the statuses of all components in the testbed (pools,
volumes, networks and domains).
improve logic in arguments coherency checks

Less code for better handling of both optional and required parameters
of each action.