Replace dir chown/chmod SAs with modify actions.

ShellActions were unnecessarily heavyweight, and have a system-specific
semantics. In case the user does something strange (replaces chown or
chmod, etc) it is much safer to use builtin python functions.
Added execute.create actions to mkdirs and files

Instead of using touch and mkdir -p, we can use the os module and an
empty file write operation. This eliminates the last need for SAs in
the current implementation of SALVE (but we do not remove them because
they may be needed later).
Made the changes to use these in dir and file blocks.
Fixed file and dir create test failures.

Mostly involves conversion of tests to support the new types of
created actions. Also had to reduce min_attrs for directory blocks.
Merge branch 'refactor/no-shell-actions' into dev