Remove full paths to binaries in dbfixtures.conf

[robmessick]

The "dbfixtures.conf" file contains full paths to the the server binaries (e.g. /usr/bin/redis-server). The defaults should be changed to the binary name (redis-server). pytest-dbfixtures should trust that the user has the PATH configured correctly in their test execution environment.

[pwilczynskiclearcode]

Ok, we'll change it into: e.g. take redis from PATH if not specified precisely in fixture or yaml configuration.

[fizyk]

Will be dealt by #160 split packages will have config options for command line and pytest.ini, and everyone will be able to change executable easily.