ui: CephInstallWizard: make first mon node static

ui: CephInstallWizard: make first mon node static

Removes the possibility to select the node on which to create the first
monitor in the configuration / initialization step and always sets it to
the current node.

This prevents that a user might select another node on which the Ceph
packages have not yet been installed. If a user did that, they would get
an error, but the Ceph config file would have been written. If the user
then does not select a valid node to create the first mon, but aborts
the wizard, they are greeted with a rados_connect error because the
config file exists, but it does not contain any mon infos that are
needed to connect to the Ceph cluster.

Creating a mon manually will remedy such a situation, but especially for
new users, this behavior is not ideal and confusing.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>