Fix backup/rotation with multiple excluded databases

* When using multiple excluded databases, the list of databases
  is filtered using `grep -v`.
  i.e. `grep -v '^\(information_schema|performance_schema\)$`
* When using Basic vs Extended Regular Expressions, the characters
  `(` and `|` lose their special meaning, the backslashed versions
  have to be used.
  For the group (`()`) the escaping has been done, however the
  alternation is unescaped.

Leading to:

* All the excluded databases will be backed up.
* In case a database is not backuppable (which is why it had been
  excluded), this leads to the cleanup not being run at all, as it
  depends on the backup having been successful.

This MR aims to fix this issue, by revising the regular expression
and specifying that behaviour in the respective class spec.

spec/classes/mysql_backup_mysqldump_spec.rb

@@ -76,13 +76,13 @@ class { 'mysql::server': }
         let(:params) do
           {
             'file_per_database' => true,
-            'excludedatabases' => ['information_schema']
+            'excludedatabases' => ['information_schema', 'performance_schema']
           }.merge(default_params)
         end
 
         it {
           expect(subject).to contain_file('mysqlbackup.sh').with_content(
-            %r{information_schema},
+            %r{information_schema\\\|performance_schema},
           )
         }
       end

templates/mysqlbackup.sh.epp

@@ -84,7 +84,7 @@ cleanup
 <% if $excludedatabases.empty { -%>
 mysql --defaults-extra-file=$TMPFILE -s -r -N -e 'SHOW DATABASES' | while read dbname
 <%} else {-%>
-mysql --defaults-extra-file=$TMPFILE -s -r -N -e 'SHOW DATABASES' | grep -v '^\(<%= $excludedatabases.join('|') %>\)$' | while read dbname
+mysql --defaults-extra-file=$TMPFILE -s -r -N -e 'SHOW DATABASES' | grep -v '^\(<%= $excludedatabases.join('\\|') %>\)$' | while read dbname
 <% } -%>
 do
   <%= $backupmethod %> --defaults-extra-file=$TMPFILE --opt --flush-logs --single-transaction \