PDU::pause: Better pause time configuration

PDU::pause: Better pause time configuration

If no time is specified, use 120 (or the PowerOnTime host property
value) when powering on, or 10 when powering off.

This is all intended to be used like this:
  HostProp_rimava0_PowerMethod nest; msw --apc6 pdu2 19; pause; ipmi rimava0m

This means:
  * For power on, first turn on the APC PDU pdu2 port 19
  * Wait 120s (or HostProp_rimava0_PowerOnTime)
  * Then ipmi to the host name rimava0m with ADMIN/ADMIN
    (or HostProp_rimava0_IpmiUser/HostProp_rimava0_IpmiPassword)
  * For power off, do that in reverse, only with a 10s pause.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>