ASM-1306 include optional erb in esxi ks.cfg

In the ESXi ks.cfg.erg, include an optional erb template based on the
server serial_number if it exists. Allows additional configuration on
a per-server basis. Currently planned to be used to set a static
management address on the host.

This is a stop-gap measure until ASM can upgrade to the latest version
of razor, which allows passing arbitrary data to the policy which can
be rendered in the erb files.