Merge tag 'pallet-0.8.0-beta.4' into develop

pallet-0.8.0-beta.4
Updated version for next release cycle
Add image-user to node-list nodes
Update to latest stevedore
Add pallet.script.lib/file like c.java.io/file

Adds a file function for concatenating path elements in script.
Add debconf-set-selections action

The action can be used to set debconf selection values.  It runs before any
`package` action.
Add :preseeds option to :packages install strategy

The :preseeds key takes a sequence of debconf-set-selections maps.
Add exclusion to prevent Enlive from checking every Clojure version.
Remove outdated launch script generator
Remove some reflection warnings
Remove unused cache implementation
More reflection warnings
Remove unused get-session function
Update script-exec, pallet-common and fsmop
Remove reflection warnings
Correct groupid in readme
Update to thread-expr 1.3.0

This is identical to thread-expr 1.2.0, but released to clojars with the
com.palletops group-id.
When a task fails to read pallet.clj, show reason

The exception thrown when pallet.clj failed to be read was being swallowed.  It
is now logged at FATAL level before aborting.
Fix argument order in spec-from-project call
Correct the fixup of md5 files with paths