Fix openldap_password function to avoid errors with latest Puppet 7

Errors:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, failed to coerce org.jruby.RubySymbol to java.lang.String

Stack trace on puppetserver:

/etc/puppetlabs/code/environments/test/modules/openldap/lib/puppet/functions/openldap_password.rb:38:in `generate_password'

lib/puppet/functions/openldap_password.rb

@@ -26,16 +26,16 @@
   def generate_password(secret, scheme = 'SSHA')
     case scheme[%r{([A-Z,0-9]+)}, 1]
     when 'CRYPT'
-      salt = call_function(:fqdn_rand_string, 2)
+      salt = call_function('fqdn_rand_string', 2)
       password = "{CRYPT}#{secret.crypt(salt)}"
     when 'MD5'
       password = "{MD5}#{Digest::MD5.hexdigest(secret)}"
     when 'SMD5'
-      salt = call_function(:fqdn_rand_string, 8)
+      salt = call_function('fqdn_rand_string', 8)
       md5_hash_with_salt = "#{Digest::MD5.digest(secret + salt)}#{salt}"
       password = "{SMD5}#{[md5_hash_with_salt].pack('m').delete("\n")}"
     when 'SSHA'
-      salt = call_function(:fqdn_rand_string, 8)
+      salt = call_function('fqdn_rand_string', 8)
       password = "{SSHA}#{Base64.encode64("#{Digest::SHA1.digest(secret + salt)}#{salt}").chomp}"
     when 'SHA'
       password = "{SHA}#{Digest::SHA1.hexdigest(secret)}"