MT#15261 Allow using symbolic links for YML files

At the moment it is impossible to use symbolic links for YML files.
The following error happens:
> root@spce:/etc/ngcp-config# ngcpcfg apply
> Error: Encoding check of /etc/ngcp-config/config.spce.yml fails: neither ASCII nor UTF-8.
> Error: Please convert /etc/ngcp-config/config.spce.yml to UTF-8.
>
> NOTE:
> * Check encoding via:
> # file /etc/ngcp-config/config.spce.yml
> * To convert ISO-8859/latin1 to UTF-8 execute:
> # iconv -f latin1 -t utf8 < /etc/ngcp-config/config.spce.yml > /etc/ngcp-config/config.spce.yml.tmp \
> && mv /etc/ngcp-config/config.spce.yml.tmp /etc/ngcp-config/config.spce.yml

As a result the following usecase ('node-type' specific file) failed:
> >>> ls -la /etc/ngcp-config/*
> total 72
> drwxr-xr-x 2 root root  4096 Sep 18 13:57 .
> drwxr-xr-x 6 root root  4096 Sep 16 23:34 ..
> lrwxrwxrwx 1 root root    15 Sep 18 13:57 config.db01a.yml -> ./config.db.yml
> lrwxrwxrwx 1 root root    15 Sep 18 13:57 config.db01b.yml -> ./config.db.yml
> -rw-r--r-- 1 root root   224 Sep 18 12:43 config.db.yml
> -rw-r--r-- 1 root root   250 Sep 18 12:34 config.lb.yml
> -rw-r--r-- 1 root root   146 Sep 18 12:44 config.mgmt.yml
> -rw-r--r-- 1 root root   251 Sep 18 13:56 config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx01a.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx01b.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx02a.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx02b.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx03a.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx03b.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx04a.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx04b.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx05a.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    18 Sep 18 13:57 config.prx05b.yml -> ./config.proxy.yml
> lrwxrwxrwx 1 root root    15 Sep 18 13:57 config.slb01a.yml -> ./config.lb.yml
> lrwxrwxrwx 1 root root    15 Sep 18 13:57 config.slb01b.yml -> ./config.lb.yml
> lrwxrwxrwx 1 root root    17 Sep 18 13:57 config.web01a.yml -> ./config.mgmt.yml
> lrwxrwxrwx 1 root root    17 Sep 18 13:57 config.web01b.yml -> ./config.mgmt.yml
> -rw-r--r-- 1 root root 13051 Sep 18 12:54 config.yml
> -rw-r--r-- 1 root root  4633 Sep 15 16:13 constants.yml
> -rw-r--r-- 1 root root 16534 Sep 15 16:09 network.yml

> >>> cat config.lb.yml
> ---
> checktools:
>   collcheck:
>     loadlong: 2
>     loadmedium: 2
>     loadshort: 3
> heartbeat:
>   pingnodes:
>     - 1.1.1.1
>     - 2.2.2.2

Change-Id: I43045d08c715d17612617c29cf1ed04cf13e650b

scripts/check

@@ -19,7 +19,7 @@ fi
 check_config_encoding() {
   log_debug "Checking encoding for $config_files"
   for f in $config_files ; do
-    if [ -r "$f" ] && ! file "$f" | grep -qe "UTF-8" -qe "ASCII" ; then
+    if [ -r "$f" ] && ! file -L "$f" | grep -qe "UTF-8" -qe "ASCII" ; then
       log_error "Encoding check of ${f} fails: neither ASCII nor UTF-8."
       log_error "Please convert ${f} to UTF-8."
       log_info