Check if the node exists in Observium database before it is added

This change allows spamless Puppet runs on the Observium server. The new
"onlyif" parameter for the Exec resource that adds a new node assumes that the
Observium database name is "observium". This can be parameterized later, if
deemed necessary. The code also depends on /root/.my.cnf having been configured
properly to allow mysql logins without entering credentials.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>