Reword 'isolated' network description

Currently the "isolated" network type description says that
this type of network will have "no physical network connection,"
which sounds like a NIC won't be created in the VM.
VMs on an isolated network will have an adapter that gets
an IP which will be only able to communicate with other VMs
in the same isolated network.

Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>