hooks: Introduce snf-network-reconfigure

This hook should run on instance-modify-post phase and
will reconfigure the VM's taps based on certain tags.
Just like with firewalling we first check if the instance has
a tag with prefix synnefo:network:<some_nic_ident>:reconfigure.
If the identifier is missing then the reconfiguration will
apply to all NICs. This hook will run only in primary node
of the instance.

Since hooks and runtime environment may differ (due to modifications
without hotplug), we get all the info stored locally. That
is /var/lib/snf-network/<TAP>-env and
/var/run/ganeti/kvm-hypervisor/nics/<INSTANCE>/<NIC_UUID>.

Then we invoke /etc/ganeti/kvm-ifup-custom with the RECONFIGURE
environment variable set to true. If this is set it sources
the already existing env file (and overrides and other environment
variables).

This feature is useful when we want to apply changes that affect
only the hosts environment and not the VM, e.g. firewalls.
To this end we do:

 # gnt-instance add-tags instance synnefo:network:0:protected
 # gnt-instance add-tags instance synnefo:network:0:reconfigure
 # gnt-instance modify --net 0: instance

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Support debug mode

If DEBUG=true in /etc/default/snf-network then all
commands will be logged (set -x) and stdout/stderr will
be redirected to /var/log/ganeti/snf-network.debug file.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>