Add more control over network configs

[prwhitaker]

Following the experience of using images created for newer versions of VCF, this PR introduces more control over the network setup that the esxi_netinit python code will generate.

It makes the following changes:

• default to only creating single vSwitch, portgroup & vmkernel, using the default VMware names for those items (gives out of the box compatibility with VCF 9)
• allow extra configuration to be controlled via userdata (whether to create additional network config & custom names for the management network items)

It also fixes a bug where the ESXi kickstart config was intended to reboot the host after executing the esxi_netinit code, but failed to do so (which leaves hosts in a state where when they are subsequently rebooted, they lose all network config)