filestorage: Enable the Rename action
filestorage: Enable the Rename action

Enable the 'Rename' function, which practically moves a file from a given path
to a new one. Also, add the necessary checks to ensure that the new path is
acceptable.

Signed-off-by: Alex Pyrgiotis <6383a799dda2258eec8fa26a3ce2ec8a7d552341@grnet.gr>
Signed-off-by: Aaron Karper <99cc1031546340b2b8ced5fe338b5890677a56ca@google.com>
Reviewed-by: Aaron Karper <99cc1031546340b2b8ced5fe338b5890677a56ca@google.com>

onponomarev/ganeti,yiannist/ganeti,dimara/ganeti,leshchevds/ganeti,andir/ganeti,bitemyapp/ganeti,ganeti/ganeti,dimara/ganeti,mbakke/ganeti,grnet/snf-ganeti,onponomarev/ganeti,mbakke/ganeti,leshchevds/ganeti,andir/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,yiannist/ganeti,ganeti/ganeti,mbakke/ganeti,grnet/snf-ganeti,bitemyapp/ganeti