support energy metrics in load-watcher

[jpedro1992]

I would like to add additional metrics to load-watcher based on Scaphandre and Kepler monitoring tools.

Example:

promScaphHostPower           = "scaph_host_power_microwatts"
promScaphHostJoules          = "scaph_host_energy_microjoules"
promKeplerHostCoreJoules     = "kepler_node_core_joules_total"
promKeplerHostUncoreJoules   = "kepler_node_uncore_joules_total"
promKeplerHostDRAMJoules     = "kepler_node_dram_joules_total"
promKeplerHostPackageJoules  = "kepler_node_package_joules_total"
promKeplerHostOtherJoules    = "kepler_node_other_joules_total"
promKeplerHostGPUJoules      = "kepler_node_gpu_joules_total"
promKeplerHostPlatformJoules = "kepler_node_platform_joules_total"
promKeplerHostEnergyStat     = "kepler_node_energy_stat"

I will submit a PR related to this issue.