Expose samba schema option. Enable domain logons = yes when LDAP enabled

- It turns out that checking for the existence of a samba schema in an LDAP
  directory is not as easy (or even possible) as one might think. If an
  LDAP directory has the samba schema installed, but no attributes
  populated, you can do a search for samba attributes and get back zero
  results, yet the schema is indeed installed!  If the LDAP directory
  has it's configuration stored in cn=schema, the credentials for that
  may vary from what is alloawed to access the server and might not even
  have permission to view the schema anyway. The easy button here is
  just to have a clickable option until I can figure out a way to probe
  for the schema, if it's even possible.

- Another issue that's been around forever is when samba joins an LDAP
  directory, if domain logons is set to no, then the hostname of the
  samba machine is added as the sambaDomainName. If domain logons is set
  to yes, the right thing is done and the workgroup value is set to the
  sambaDomainName. I'm not sure why this behavior occurs, however, I am
  setting domain logons to yes when ldap is enabled to force samba to do
  the right thing.