Support uploading multiple certificates for ldap configuration

This code change regards replacing a path to CA file with directory
location holding multiple CA files within it.

Implementation assumes that one can still define TLS_CACERT_FILE as
either a single CA file or directory location.
Depending if the path points to a file or a directory a proper
value will be set in /etc/nslcd.conf

This code change depends on another change requests:
https://gerrit.openbmc-project.xyz/c/openbmc/meta-phosphor/+/25987
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-certificate-manager/+/23348

Tested:
   Manually tested, all changes propagate properly to
   /etc/nslcd.conf file.
   Unit Tests are passing.

Signed-off-by: Zbigniew Kurzynski <zbigniew.kurzynski@intel.com>
Depends-On: Icd33723c1fc2580679aaaf54b3e99dfb09342402
Depends-On: Ia02c552eb27744e45ccfff3b3a1232d10e65da74
Change-Id: I85dabd4841018f04b0b9e9b58dca9579e7ff1999
4 files changed