Unable to add server to existing deployment through UI

Bug description

The add to existing deployment button is removed and the installation steps for this scenario are changed. Documentation states:

 In the configuration store screen, ensure that you configure the same DS instance that is already working as the configuration store for the rest of the instances on the site.

https://backstage.forgerock.com/docs/am/7/install-guide/configure-sites.html#add-servers-to-site

If you follow these steps AM is complaining that: Config data already exists under root suffix in this host

Which is true because this configuration store is already configured by the first AM node. 

How to reproduce the issue

1. Install external DS server and set the configuration, identity, CTS store profile.
2. Install a AM node
3. Install a second AM node and point it to the same configuration store as stated in the documentation. Note that you use the same encryption key as for the first AM server.

Expected behaviour

Continue with the configuration and adds the second AM node to the existing deployment.

Current behaviour

A warning is showed in the screen were you configured your configuration store: Config data already exists under root suffix in this host
The "Next" button is greyed out and you are not able to continue the installation.

Work around

The current workaround is to do a silent installation of AM to bypass this bug.

https://backstage.forgerock.com/docs/am/7/install-guide/silently-install-openam.html