Remove ramdisks for RRD databases

Ramdisks are very limited in space and as new graphs
are generated for OpenVPN N2N connections, etc. more
space is necessary.

This patch will enable ramdisks for all systems with more
than 490M of memory and allows the user to force using
a ramdisk on systems with less memory.

Signed-off-by: Alexander Marx <alexander.marx@ipfire.org>
Acked-by: Arne Fitzenreiter <arne.fitzenreiter@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>