fix #1683: do not send non-numeric values to graphite

fix #1683: do not send non-numeric values to graphite

the graphite daemons which accept the data (carbon), only
accepts numeric values, and logs all invalid lines

since that were about 5 values per vm/ct this generated lot of noise
in the carbon log

so we check with a regex if a value is numeric, and
additionally we have a blacklist of keys which seem to be numeric but
are either boolean (e.g. template) or a state (e.g. pid)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>