populate_dir: only makedir if it doesn't exist
add 'pathprefix2dict' utility for use by DataSourceNoCloud
DataSourceNoCloud: support reading vendor-data 

Here we add the ability to read vendor-data from a file named
vendor-data at the same location as the user-data and meta-data files.

At the moment, vendor-data is not read at all from 'seedfrom'.
cloudinit/util.py: fix pylint complaints
add 2 tests for vendordata
merge from trunk
DataSourceNoCloud: support reading vendor-data 

Here we add the ability to read vendor-data from a file named
vendor-data at the same location as the user-data and meta-data files.

At the moment, vendor-data is not read at all from 'seedfrom'.