Allow sshd to pull ssh keys from ldap on Trusty.

This has three parts:

modules/ldap/files/scripts/ldapkeys:  A simple ldap query tool
 (based off of ldaplist) which takes a username and looks up
 that user's public ssh keys in ldap, using the ldap config
 in ldap.conf.

modules/ldap/manifests/client.pp:  Installs the ldapkeys script
 where appropriate, along with a service user also named ldapkeys
 which ssh will use to invoke the script.

modules/ssh/templates/sshd_config.erb:  Specifies use of the
  ldapkeys tool and the ldapkeys user for ssh attempts.

Change-Id: I622d4807012a1fc935d6bc197adc266fb4cb21df
Merge "Allow sshd to pull ssh keys from ldap on Trusty." into production