Change migration functions to take a config map instead of file

Currently, the run-migrations and run-rollbacks functions take config files and read in the
edn themselves. This does not allow support for runtime modification of config files, as provided
by boot.clj. Changing the migration functions to take a map allows the lein-caribou plugin to provide
a map via a config-file as before, but also a "dynamic" config by specifying a boot function that
generates a config map. See pull request in lein-caribou on branch boot-config-support.