nvagent: use '-k' instead of '-p' to specify the provisioning key
nvagent: add an option '-p' to select a profile

A profile is only a directory ~/.netvirt/PROFILE/ where all the
files (certificates, IP and conf file) are placed after the agent
is being provisioned. It is now possible to provision the agent
under a specific profile, and then connect to this profile by
simply specifying its name.