Use templates for power calls

[mslacken]

Refactor the wwctl power calls in such way that ipmitool is not hardcoded,
but is deducted from the ipmitool.tmpl which is located in the $DATADIR
This empowers users to have other memthods for the power managemnt like
freeipmi or redfish without touching the warewulf code itself. As an
example a template for power management without bmc is included.

1. @anderbubble don't hit the merge button, because test coverage has
   increased a lot, but using the template is one of my best ideas in
   the last time
2. @griznog we can use the template approach also for wwctl container copy
   so that on zfs pools the directory is cloned and on btrfs a cp --reflink=auto
   is used.

[griznog]

This is an awesome approach! I am very much in love with this idea and would like to see as many wwctl commands as possible have the option to be calls to a template.