ceph-ansible-syntax: display matched files in group_vars_check

ceph-ansible-syntax: display matched files in group_vars_check

it can be useful to see which files have matched in `group_vars_check()`
function.
I'm currently facing an issue where the CI complains on that check while
it doesn't on an other environment when trying to reproduce the issue.

on the CI:

```
+ group_vars_check
++ match_file /defaults/main.yml
++ git_diff_to_head
++ wc -l
++ git diff origin/master..HEAD -- . ':(exclude)roles/*/meta/*'
++ sed -n 's|^+++.*\(/defaults/main.yml.*\)|\1|p'
+ nb=1
+ [[ 1 -eq 0 ]]
++ match_file group_vars/
++ wc -l
++ git_diff_to_head
++ git diff origin/master..HEAD -- . ':(exclude)roles/*/meta/*'
++ sed -n 's|^+++.*\(group_vars/.*\)|\1|p'
+ nb_group_vars=0
+ [[ 1 -gt 0 ]]
+ echo 'One or more files containing default variables has/have been modified.'
One or more files containing default variables has/have been modified.
+ echo 'You must run '\''generate_group_vars_sample.sh'\'' to generate the group_vars template files.'
You must run 'generate_group_vars_sample.sh' to generate the group_vars template files.
+ return 1
```

local env:

```
+ group_vars_check
++ match_file /defaults/main.yml
++ wc -l
++ git_diff_to_head
++ sed -n 's|^+++.*\(/defaults/main.yml.*\)|\1|p'
++ git diff origin/master..HEAD -- . ':(exclude)roles/*/meta/*'
+ nb=0
+ [[ 0 -eq 0 ]]
+ echo 'group_vars has not been touched.'
group_vars has not been touched.
+ return 0
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>