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.
Merge pull request #7 from gavin-scott/asm-1306

ASM-1306 include optional erb in esxi ks.cfg