update pulpcore and pulp_file functional tests to use a configurable hostname+prefix for fixtures
Pulp-smash config needs to accept a 'fixtures_origin' setting. This should default to 'https://repos.fedorapeople.org/repos/pulp/pulp/fixtures/'.
All the functional tests in pulpcore and pulp_file need to be updated to use 'fixtures_origin' config when constructing URLs for File Remotes to sync from.
The Travis scripts for pulpcore and pulp_file need to set the 'fixtures_origin' setting to 'http://localhost/' before running the functional tests.