core: adding retry for host start for PPC

Sometimes a restart operation started by engine is not completed because
the host PM failed to put the host on 'on' state
This occurred on PPC hosts only until now.
Some fencing scripts as the one used here (IPMI) have an option to se
retries for the 'on' operation by sending : retry_on=<number>

The patch adds "retry_on=2" for the relevant PPC fencing scripts such
that this will be sent by engine as a default when a PPC host has one of
those agents used

Change-Id: I2c7a195526eb3f6454116806131294de2d3fc067
Signed-off-by: Eli Mesika <emesika@redhat.com>