(NETDEV-20) Change snmp_notification_receiver to use composite namevar

Without this patch the snmp_notification_receiver specifies the name
parameter as the sole namevar.  This is a problem because there can be
multiple resources with the same hostname or IP address.  This patch
addresses the problem by changing the type to build a composite namevar
that joins hostname, port, version, and type with a colon to form the
resource identifier.  This looks like:

    $ puppet resource snmp_notification_receiver
    snmp_notification_receiver { '127.0.0.1:public:162:v3:traps:noauth':
      ensure   => 'present',
      port     => '162',
      security => 'noauth',
      type     => 'traps',
      username => 'public',
      version  => 'v3',
    }
    snmp_notification_receiver { '127.0.0.1:smtpuser:162:v3:traps:auth':
      ensure   => 'present',
      port     => '162',
      security => 'auth',
      type     => 'traps',
      username => 'smtpuser',
      version  => 'v3',
    }
    snmp_notification_receiver { '127.0.0.2:private:162:v2:traps':
      ensure    => 'present',
      community => 'private',
      port      => '162',
      type      => 'traps',
      version   => 'v2',
    }
    snmp_notification_receiver { '127.0.0.3:public:162:v1:traps':
      ensure    => 'present',
      community => 'public',
      port      => '162',
      type      => 'traps',
      version   => 'v1',
    }
    snmp_notification_receiver { '127.0.0.4:priv@te:162:v1:traps':
      ensure    => 'present',
      community => 'priv@te',
      port      => '162',
      type      => 'traps',
      version   => 'v1',
    }
    snmp_notification_receiver { '127.0.0.4:private:10162:v2:informs':
      ensure    => 'present',
      community => 'private',
      port      => '10162',
      type      => 'informs',
      version   => 'v2',
    }
    snmp_notification_receiver { '127.0.0.4:private:20162:v1:traps':
      ensure    => 'present',
      community => 'private',
      port      => '20162',
      type      => 'traps',
      version   => 'v1',
    }
    snmp_notification_receiver { '127.0.0.4:public:162:v1:traps':
      ensure    => 'present',
      community => 'public',
      port      => '162',
      type      => 'traps',
      version   => 'v1',
    }