Hide cisco switch unless really need

Only show the cisco switch UI when the cisco plugin is selected and the mode is
set to VLAN
Only show the Cisco topology editor if there Cisco support is enabled in the proposal
Remove "cisco_support" key it's no longer needed
Fix VLAN mode for cisco plugin

- Write ssh passwords to credentials.ini
- Fix creating of nexus.ini (switchports and driver setting)
Don't try to process the nexus switch config in gre mode

For the gre mode (fake driver) no switches need to be defined
The cisco nexus plugin needs a tenant setting in the credentials

This requirement  was added to grizzly recently (2013.3 release). See:
https://bugs.launchpad.net/neutron/+bug/1202462