Fix the installation location of mon-collector

The mon-collector binary was erroneously installed twice: once in
/usr/bin/ and once in /usr/lib/ganeti/.
With this modification, it is now installed in /usr/lib/ganeti/ only,
as intended.

In order to do this in a proper way, some parts of the Makefile.am where
refactored.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>