engine: Add Label and Unlabel network commands

The commands are added for labeling and unlabeling
a network. The commands will be used from the rest
api where the labels are a sub-collection of the
network.

Change-Id: I2064d24f0d357ca8d477266e4a9e18939bf21ad0
Signed-off-by: Moti Asayag <masayag@redhat.com>