[User guide]Add `fixed-address` argument to openstack server command
[User guide]Add `fixed-address` argument to openstack server command

* Adding the --fixed-address argument to the openstack server command
in the user guide following conversion from the nova command.

Change-Id: I911b2a47ca7b99d4324fa46db1894e08fc4e4e5f
Closes-bug: #1649611

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals