handle exception when input tpl could not be read

When the input template file could not be read by cloubed, an IOError
exception is raised by Python. cloubed now handles it to print to warn
the user properly.

Fixes #28.
detect conflicts on network creation

Before a network is created in Libvirt, cloubed now checks is there is
another active network with conflicting IP settings. If yes, a
CloubedException is raised in the library and the script prints the
error nicely along with the name of this boring network.

IP networks comparaison is done with the help of the netaddr library.
Therefore, a dependency has been added to this external library.

Fixes #30.