Clean up exec_dirs prefix from rootwrap conf

Specifying the full path to binaries in exec_dirs
is not necessary and actually causes a problem if
the binary is in a different path. The rootwrap
infrastructure allows the binaries to be in all
paths specified by exec_dirs.

Change-Id: Ic16a779e916cd3b727db54b034d069c122aa45df
Merge "Clean up exec_dirs prefix from rootwrap conf"