Previously, installation was being forced on the source directory
itself.  Python code itself required it to import the pyclient and
proto packages/directories.

This commit changes so that we're installed on CMAKE_INSTALL_PREFIX
which has a standard default and can be set by the user.  The python
packages are installed in DATADIR (since it's platform independent)
and the legacy_serve python script, renamed to cpuvisor_legacy_serve,
is installed in BINDIR being generated on the fly during the configure
step.  Out of source builds are recommended and gitignore will no
longer ignore files installed on the source directory.