hypervisor: manage nova shell

To enable live-migration & resize feature, nova user needs to run
/bin/bash shell (need from libvirt).
By default packaging set /bin/nologin for some security reasons.
Let's bring more flexibility and let the end user to set another shell
for Nova on Hypervisor nodes.
Defaults to false to keep default packaging configuration.

Close #600
Merge pull request #647 from enovance/nova-user

hypervisor: manage nova shell