Allow to customize search filter for LDAP. Fixes #CAS-1482

[ copied from https://issues.jasig.org/browse/CAS-1482 ]
Our corporate Active Directory has users migrated to another domain. As a result, it is necessary to bind to LDAP with DN including domain name (like `user@COMPANY`), but to perform the search with user name only. I created a patch, which adds property `searchUserOnly` which defaults to false to keep the current behavior. Also, I created a property `searchFilter` to override the current string: 

    userPrincipalName={0}

with search filter I had to use for our Active Directory:

    (&(objectClass=user)(sAMAccountName={0}))

So I can define bean like:

    <bean id="entryResolver"
      class="org.jasig.cas.authentication.support.UpnSearchEntryResolver"
      p:baseDn="${ldap.baseDn}"
      p:searchUserOnly="true"
      p:searchFilter="${ldap.searchFilter}"/>

in a property file I have ldap.searchFilter specified as:

    ldap.searchFilter=(&(objectClass=user)(sAMAccountName={0}))