ceph-iscsi: Update gateway config/template

ceph-iscsi: Update gateway config/template

- Remove gateway_keyring from the configuration file because it's
not used in ceph-iscsi 3.x release.
- Use config_template instead of template module for iscsi-gateway
configuration file. Because the file is an ini file and we might want
to override more parameters than those present in ceph-ansible.
- Because we can now set the pool name in the configuration, we should
use a variable for that. This is refact with the iscsi_pool_* variables
also used to configure the pool size.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit 1f2a4f1910d75241deb345228227a3ad7711c145)