In order to have a hostname passed as a configuration param when
creating an allocation network, the CreateNetwork func of the
DriverNetworkManager interface needs to be updated. In order to
minimize the disruption of future changes, rather than add another
string func arg, the function now accepts a request struct. The
struct has the hostname as a field.

The in-tree implementations of DriverNetworkManager.CreateNetwork
have been modified to account for the function signature change.
In updating for the change, the enhancement of adding hostnames to
network namespaces has also been added to the Docker driver, whilst
the default Linux manager does not current impelement it.

The allocrunner includes additional validation of the new parameter
to ensure it is used in the correct situations.

Add a new hostname string parameter to the network block which
allows operators to specify the hostname of the network namespace.

Changing this causes a destructive update to the allocation and it
is omitted if empty from API responses.