Only search the lib dir for rpc_backend

The grep in rpc_backend uses . to search all files. Unfortunately
the current directory when the command is called is not the lib
directory, so change it to search the lib directory instead.
Without this fix the command is forced to grep all files under
devstack which could be very slow if one has downloaded large
images.

Fixes bug 1174718

Change-Id: Ic8a59885cd07e5d6f2e734e80824762f9ac12996
Merge "Only search the lib dir for rpc_backend"