ipmi.py sends ipmi metrics with forward-slash in their names

[hakansonOHSU]

This report applies to ganglia 3.7.2 as distributed with CentOS-7.5.

Many errors appear in the logs like this:

gmetad: RRD_create: creating '/var/lib/ganglia/rrds/__SummaryInfo__/ipmi_bb_1_8v_ib_i/o.rrd': No such file or directory
gmetad: Unable to write meta data for metric ipmi_bb_1_8v_ib_i/o to RRD

I'm no Python programmer, but it looks like the routine mangle_metric_name needs a .replace("/","_") added to its last return statement.

[hakansonOHSU]

Or maybe it would be cleared up by a fix for ganglia/monitor-core#265