define dependency handling

Signed-off-by: Nico Schottelius <nico@kr.ethz.ch>

doc/dev/logs/2011-03-15

@@ -0,0 +1,27 @@
+Steven, Nico
+
+How to handle dependencies:
+
+1) Add --require parameter for all types
+   - Special handling in cdist-type-emulator
+   + Everything on one line
+
+2) Add __require type
+   + No change in core
+   - Type influences core
+   - Additional line
+   - Core needs to know about requirements
+
+3) cdist-require as a seperate executable
+   + No change in cdist-type-emulator
+   - new behaviour
+   - first time cdist-xxx dependency in types
+
+4) require="" environment variable for cdist-type-emulator
+   + on one line / same context
+   + special handling is ok for special case
+   + doesn't touch parameters (i.e. type still has full control)
+
+--------------------------------------------------------------------------------
+
+Result: Use version 4.