Make ironic's service check flexible

We currently assume we are deploying ironic with the rest of
a cloud and assert that glance/neutron/nova are enabled. This
makes it a bit more flexible and allows deploying with only
the minimum required services if desired, and asserts the others
are enabled when we intend on testing nova+ironic integration.

This is required for in-tree python-ironicclient functional tests,
which we aim to run against a minimal devstack deployment.

Change-Id: I99001d151161fa225b97c3ba6b167a61aa9b59fe
diff --git a/lib/nova_plugins/hypervisor-ironic b/lib/nova_plugins/hypervisor-ironic
index 0169d73..b9e286d 100644
--- a/lib/nova_plugins/hypervisor-ironic
+++ b/lib/nova_plugins/hypervisor-ironic
@@ -54,9 +54,7 @@
 
 # install_nova_hypervisor() - Install external components
 function install_nova_hypervisor {
-    if ! is_service_enabled neutron; then
-        die $LINENO "Neutron should be enabled for usage of the Ironic Nova driver."
-    elif is_ironic_hardware; then
+    if is_ironic_hardware; then
         return
     fi
     install_libvirt