Missing configuration for Trove single tenant

The nova_proxy_* options are not needed when Trove single tenant
functionnality is not used.

The current way how Kolla configure trove-taskmanager.conf is to use the
user tenant so the nova_proxy_* options are not required by default.

I added the "enable_trove_singletenant" option to enable the single tenant
functionnality if required and complete the configuration to make it works.

When enable_trove_singletenant is true, the below configuration will be
applied to trove-task-manager.conf configuration file:

nova_proxy_admin_pass = {{ trove_keystone_password }}
nova_proxy_admin_tenant_name = services
nova_proxy_admin_user = trove
remote_nova_client = \
    trove.common.single_tenant_remote.nova_client_trove_admin
remote_cinder_client = \
    trove.common.single_tenant_remote.cinder_client_trove_admin
remote_neutron_client = \
    trove.common.single_tenant_remote.neutron_client_trove_admin

Change-Id: I9858acd9486a3f6a07c1edad14fde12f49df772b
Closes-Bug: #1743394