Work around lein bug with : in path name

Lein cannot handle : in path names
(https://github.com/technomancy/leiningen/issues/891), so this commit works
around the problem by using _ in the matrix definition. Jenkins then creates
directories in the workspace with _ in the name, which lein handles fine. To
ensure the correct task is called, the _ is replaced with : before invoking
rake.
Let's handle spaces in paths correctly.
Allow environment variables to override params file

We use COW and MOCK environment variables to specify a mock/cow different from
the default listed in the params file, so we need the enviroment variable to
correctly override the params file.
Merge pull request #162 from haus/work_around_leins_deficiencies

Work around lein bug with : in path name