Parser for filter_func and goodness_func can't handle ":" characters

When writing a filter_function in cinder.conf any capabilities key that contains a ":" character can't be used in the filter because the parser can't handle this character.

Example Valid:

filter_function = "extra.some_non_prefixed_key >= 10"

Example Invalid:

filter_function = "extra.PREFIX:some_prefixed_key >= 10"

The parser does not support index notation for dictionaries, so this is not valid either:

filter_function = "extra['PREFIX:some_prefixed_key'] >= 10" even though it is valid Python

This means any vendor prefixed extra_spec is precluded from being used in a filter_function or goodness_function. This eliminates many possibilities for selection among several backends of the same vendor.