Monkey patch `knife bootstrap` to support knife-solo

Add `--solo` command line option and `knife[:solo]` configuration
parameter to `Knife::Bootstrap` that can be used for triggering
`Knife::SoloBootstrap` instead of the normal template based chef-client
bootstrap.

This is especially useful with other knife plugins like knife-ec2 that
invoke `knife bootstrap` after creating an server instance. Even if
these plugins do not have the `--solo` option, you can put
`knife[:solo] = true` in knife.rb.
Add integration test for `knife bootstrap --solo`

Convert Debian 7 test for this.
Document knife bootstrap integration