Small updates to the README
Small updates to the README

This patch updates the name of the plugins where they changed
and documents the features of the supported cloud-config format,
including write_files and set_timezone directives.

Change-Id: I2b794bb7ec41d45d2750cd5181034fd6f9bcd582

bincentvaret/bsd-cloudinit,stefan-caraiman/cloudbase-init,ader1990/cloudbase-init,alexpilotti/cloudbase-init,pellaeon/bsd-cloudinit,pellaeon/bsd-cloudinit,CropCircleSys/bsd-cloudinit,chialiang-8/cloudbase-init,whiteear/cloudbase-init,stackforge/cloudbase-init,cmin764/cloudbase-init,openstack/cloudbase-init