LU-13225 utils: fix install path for bash-completion

LU-13225 utils: fix install path for bash-completion

Fix the default install path for bash-completion if the package is
not installed at build time.  This avoids BASH_COMPLETION_DIR being
badly formatted in the lustre.spec file.

Fixes: dfb4afc24102 ("LU-13225 utils: bash completion for lfs and lctl")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie50071c4ff86f57bc9dd53409ae339da2a3ebbe5
Reviewed-on: https://review.whamcloud.com/38548
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Ben Evans <beevans@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>