Improve the supportability of LDAP

The system should check if saslauthd is available and that the couchbase user has permission to read the file when the customer starts to config LDAP, if it does not it should give a easy to consume message in the UI. For example:

Could not find saslauthd, please make sure SASLAuthd is installed. (Link to the documentation)

Could not read the saslauthd.conf file please ensure the Couchbase user had permission (Link to the documentation)

Further more the cluster should not allow new nodes to join the cluster without meeting the requirements set above when LDAP is enabled again in the UI.

Could not add node X to the cluster has it did not have SASLAuthd is installed. Please disable ldap or install SASAuthd. (Link to the documentation)

This will give the user a much better experience and reduce the load on support.