Add slow image building warning
Add slow image building warning

Add information about slow image building on low IO
disks QEMU/KVM deployments and the possible
"provision timeout" error.

Change-Id: I4c7a68070e9d19bcb695f8b4ccf313b52c76e41e

stackforge/fuel-docs,stackforge/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs,ogelbukh/fuel-docs