ldap: Add distro feature for LDAP

LDAP related packages will be pulled into the
image depending on the distro feature.

Enabling this feature increases the image size by ~500KB
as it brings nss-pam-ldapd, openldap, and nscd into the image.

(From meta-phosphor rev: b40b3f9b8c9f362583a5edcb6a64f0d3304dad5b)

Change-Id: I92ec1d2c20adfa08a87265d85962cb0390e0c854
Signed-off-by: Ratan Gupta <ratagupt@in.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>