Add additional steps for configuring Nodepool service on CentOS 7

This change adds the additional steps to follow while installing Nodepool
on CentOS 7. The included steps are
- creation of /etc/systemd/system/nodepool-launcher.service.d to place
provided drop-in file
- copying provided drop-in file to the appropriate location so ExecStart
command can be overriden by systemd

Apart from the additional steps, the package installation command on Fedora27
is switched to yum to keep the differences between Fedora27 and CentOS 7 at
minimum.

Also some, of the nodepool configuration steps like creation of the
directories and ssh key generation are moved into nodepool_install.rst
similar to how zuul_install.rst lists some of these steps.

SSH key nodepool_rsa is needed in zuul_install.rst but it gets created later
in nodepool_openstack.rst so moving the key generation to nodepool_install.rst
prevents possible confusion as well.

Depends-On: https://review.openstack.org/#/c/564872/
Change-Id: If073ba5559d3ad055c0c8a1f2f4213ce6ed8af9f

doc/source/admin/nodepool_install.rst

@@ -3,19 +3,46 @@
 Install Nodepool
 ================
 
-::
+Initial Setup
+-------------
 
-   sudo adduser --system nodepool --home-dir /var/lib/nodepool --create-home
-   git clone https://git.zuul-ci.org/nodepool
-   cd nodepool/
-   sudo dnf -y install $(bindep -b)
-   sudo pip3 install .
+.. code-block:: console
+
+   $ sudo adduser --system nodepool --home-dir /var/lib/nodepool --create-home
+   $ ssh-keygen -t rsa -b 2048 -f nodepool_rsa  # don't enter a passphrase
+   $ sudo mkdir /etc/nodepool/
+   $ sudo mkdir /var/log/nodepool
+   $ sudo chgrp -R nodepool /var/log/nodepool/
+   $ sudo chmod 775 /var/log/nodepool/
+
+Installation
+------------
+
+.. code-block:: console
+
+   $ git clone https://git.zuul-ci.org/nodepool
+   $ cd nodepool/
+   $ sudo yum -y install $(bindep -b)
+   $ sudo pip3 install .
 
 Service File
 ------------
 
 Nodepool includes a systemd service file for nodepool-launcher in the ``etc``
-source directory. To use it, do the following steps::
+source directory. To use it, do the following steps.
 
-  $ sudo cp etc/nodepool-launcher.service /etc/systemd/system/nodepool-launcher.service
-  $ sudo chmod 0644 /etc/systemd/system/nodepool-launcher.service
+.. code-block:: console
+
+   $ sudo cp etc/nodepool-launcher.service /etc/systemd/system/nodepool-launcher.service
+   $ sudo chmod 0644 /etc/systemd/system/nodepool-launcher.service
+
+If you are installing Nodepool on ``CentOS 7`` and copied the provided service
+file in previous step, please follow the steps below to use corresponding
+systemd drop-in file so Nodepool service can be managed by systemd.
+
+.. code-block:: console
+
+   $ sudo mkdir /etc/systemd/system/nodepool-launcher.service.d
+   $ sudo cp etc/nodepool-launcher.service.d/centos.conf \
+        /etc/systemd/system/nodepool-launcher.service.d/centos.conf
+   $ sudo chmod 0644 /etc/systemd/system/nodepool-launcher.service.d/centos.conf

doc/source/admin/nodepool_openstack.rst

@@ -14,7 +14,6 @@ that will be installed when instantiating the servers::
 
    cd ~
    source <username>-openrc.sh  # this may prompt for password - enter it
-   ssh-keygen -t rsa -b 2048 -f nodepool_rsa  # don't enter a passphrase
    openstack keypair create --public-key nodepool_rsa.pub nodepool
 
 We'll use the private key later wheen configuring Zuul.  In the same
@@ -39,13 +38,6 @@ session, configure nodepool to talk to your cloud::
 Once you've written out the file, double check all the required fields
 have been filled out.
 
-::
-
-   sudo mkdir /etc/nodepool/
-   sudo mkdir /var/log/nodepool
-   sudo chgrp -R nodepool /var/log/nodepool/
-   sudo chmod 775 /var/log/nodepool/
-
 Configuration
 -------------