bashcomp: Install and package completion scripts.

bashcomp: Install and package completion scripts.

This commit makes the bash completion scripts be installed to
$(sysconfdir)/bash_completion.d/ through 'make install' and to
/etc/bash_completion.d/ through package installation.  This will
make the scripts available for each bash session.

An alternative is to put scripts to /usr/share/bash_completion/
directory.  However, this is not supported by earlier version of
bash completion.

Signed-off-by: Alex Wang <alexw@nicira.com>