add uwsgi provider for WSGI applications
use a single validation error handling function
add one role validation method to be used in child objects
use a validation_error method which also does logging
extend drop target to include the box of servers

Fixes bnc#838662
added methods for validating the existence of a db proposal
allow comma-separated lists as well as space separated ones
extending 'wait for chef' for other states
timeout must be integer
remove comment from xml config
Ignore pid files with invalid content

Vincent pointed out in the previous review that we
want to also ignore pid files with invalid content
(like e.g. 0 byte sized files)