wreck: Use flux config subsystem for runtime paths; add tests, and other fixes

Leverage the `etc/flux/config` file for wreck subsystem when running out of builddir.

This pull request does away with the ugly `wrexecinst.so` hack and uses the config group `wrexec.`
to stash paths that are dependent on whether a flux session is running out a builddir or install dir.
The paths currently used are
 * `wrexec.wrexecd_path` : path to wrexecd binary
 * `wrexec.lua_pattern`: glob pattern for loading lua plugins into wrexecd

If these config values are not found, a default (the install location) for both of these values are
used. 

Additionally, a couple tests for `wreckrun` operation are added to the testsuite under `./t`
Since these require the `hostlist` lua module, this package had to be added to the travis-ci tests.