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. This parameter also supports
interpolation.
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 along with
the allocID param. 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 implement it.
The `e2ejob` utility asserts that a job is running for 5s, but with a sleep
time of 5s, the networking job can race with that check. Sleeping for a longer
period should guarantee that we're running long enough to pass the assert.
Also constrains the job to Linux because our Windows test targets don't yet
support Docker (LCOW), and expand the set of DCs we can safely land on.
