Allow selecting the network for doing the ssh with

Previously, nova ssh was searching for network types: public and
private, which seems to be incorrect (fixed and floating seems to be
correct), causing that this command has probably never worked.

This commit fixes the above and adds an option for selecting the network
to use, which is helpful when there are more networks for the VM.

Change-Id: I01ea6cee725c0feaacab60975c3792b0ac1305e9
Closes-Bug: #1227694
Closes-Bug: #1343991
Merge "Allow selecting the network for doing the ssh with"