engine: Network Command Validation

This patch extracts repetitive code to the
NetworkCommon class, and cleans up canDoActions
in both AddNetwork and UpdateNetwork commands, to
use the Validation framework given by the
CommandBase.

Change-Id: I4bb5bafd2029b3a8729bcf78d359804c6f587ff7
Signed-off-by: Muli Salem <msalem@redhat.com>