Backslashes in a password need to be escaped

This will replace a single backslash with a double backslash in the
`/root/.mongoshrc.js` file. when a password with a backslash is used, it
is correctly passed on to the provider for setting the user's password,
but things break when attempting to use said password for the admin
user.

A small explanation on the amount of backslashes: The first argument is
a regular expression, so we need to escape the backslash. The second
argument allows for references to capture groups or the entire match
using backslashes, for example `\0` contains the entire match.  This
would make us end up with 4 backslashes, but apparantly the template
rendering also has backslash escaping, this we need to double the amount
of backslashes. So 8 in total.

spec/classes/server_spec.rb

@@ -452,6 +452,22 @@
               with_mode('0600').
               with_content(%r{admin\.auth\('admin', 'password'\)})
           }
+
+          context 'with backslash in password' do
+            let :params do
+              {
+                admin_username: 'admin',
+                admin_password: 'password_\_with_backslash',
+                auth: true,
+                store_creds: true
+              }
+            end
+
+            it {
+              is_expected.to contain_file('/root/.mongoshrc.js').
+                with_content(%r{admin\.auth\('admin', 'password_\\\\_with_backslash'\)})
+            }
+          end
         end
 
         context 'false' do

templates/mongoshrc.js.erb

@@ -34,7 +34,7 @@ if (authRequired()) {
   <%- end -%>
   try {
     admin = db.getSiblingDB('admin')
-    admin.auth('<%= @admin_username %>', '<%= @admin_password_unsensitive %>')
+    admin.auth('<%= @admin_username %>', '<%= @admin_password_unsensitive.gsub('\\','\\\\\\\\') %>')
   }
   catch (err) {
     // Silently ignore this error, we can't really do anything about it.