<mongodb cart> adapt to use SCL-provided mongodb

Bug 1073126
https://bugzilla.redhat.com/show_bug.cgi?id=1073126

Adds `lib/mongodb_context`, which wraps MongoDB executables with
appropriate SCL context

Adds `update_configuration` function: sets up `PATH`, `MANPATH`, and
`LD_LIBRARY_PATH` for shell env

Update `mongo` shell function in `rhcsh`: don't clear `LD_LIBRARY_PATH`
before calling `mongo`, trust the `PATH` variable to find the right
`mongo`, and resolve mongo path into var `mongo_bin`