dovecot.conf template: fixed auth service clients limit. Refs #1747

Fixes f597bb74509, rollbacks dovecot 2.1.1 dependency
/etc/sysconfig/dovecot template: Obtain Kerberos TGT on dovecot startup. Fixed file descriptor limit. Refs #1747
/etc/dovecot/active-directory.conf template: read LDAP setup from `net ads` output. Refs #1747
/etc/dovecot/dovecot.conf template (30ads): add auth_gssapi_hostname, auth_krb5_keytab parameters. Refs #1747
nethserver-samba-{save,update,adsjoin} events: add Active Directory LDAP as DB backend for dovecot users and postfix aliases when smb ServerRole is ADS. Refs #1747
Configuration DB defaults: kerberos setup for dovecot and postfix. Refs #1746
/etc/postfix/active-directory.cf template: LDAP table configuration for postfix >= 2.8 . Refs #1747
/etc/postfix/chroot-update template: perform kerberos initialization at postfix daemon startup. Refs #1747
*.spec.in: increased dovecot and postfix version requirements. Refs #1747
nethserver-mail-server-conf action: restart slapd cleanly. Refs #1747 #1722