Add to CentOS install guide

Add information about the private tmp dirs enabled by default. Provide
instructions for disabling private tmp dirs for HTTPD so that omni
can be invoked properly.

Add information about enabling the HTTPD and SHIBD services at boot
time. Include commands to verify that the services are enabled.

INSTALL-centos.md

@@ -4,7 +4,7 @@
 
 For installing the GENI Portal Software, shell windows on three servers are required:
 
- * The Portal host 
+ * The Portal host
  * The IdP host
  * The development host (from which the user can scp from/to the other hosts)
 
@@ -108,7 +108,7 @@ sudo cp /tmp/hosts /etc/hosts
 ```
 
 
-# 3. Install Shibboleth Software 
+# 3. Install Shibboleth Software
 
 3a. Edit shibboleth attribute-map.xml
 ```
@@ -165,11 +165,11 @@ sudo service tomcat6 restart
 
 ```
 # On development host:
-scp $IDP_HOST:/opt/shibboleth-idp/metadata/idp-metadata.xml  /tmp/idp-metadata-$IDP_HOST.xml 
+scp $IDP_HOST:/opt/shibboleth-idp/metadata/idp-metadata.xml  /tmp/idp-metadata-$IDP_HOST.xml
 scp /tmp/idp-metadata-$IDP_HOST.xml $PORTAL_HOST:/tmp
 ```
 
-``` 
+```
 # On portal host:
 # Add host-specific extensions to IDP metadata for GENI logo, name, etc.
 sed -e "/<Extensions>/r /tmp/idp-metadata-extension.xml" /tmp/idp-metadata-$IDP_HOST.xml > /tmp/idp-metadata-$IDP_HOST.extended.xml
@@ -204,7 +204,44 @@ sudo cp /tmp/portal-*.pem /usr/share/geni-ch/portal
 sudo cp /tmp/km-*.pem /usr/share/geni-ch/km
 ```
 
-# 9. Restart HTTPD service
-```bash
+# 9. Disable HTTPD private tmp directory
+
+The portal uses /tmp to communicate between the portal and launched
+omni/stitcher commands. Depending on the installation, CentOS may enable
+a private /tmp directory for httpd which will hide the necessary files
+from launched omni/stitcher processes.
+
+To disable private tmp directory for httpd, edit the file:
+
+    /etc/systemd/system/multi-user.target.wants/httpd.service
+
+and set `PrivateTmp` to false:
+
+````
+PrivateTmp=false
+````
+
+# 10. Enable HTTPD and SHIBD services to start at boot time
+
+The following commands will enable the services to start at boot time:
+
+````sh
+sudo systemctl enable httpd.service
+
+sudo systemctl enable shibd.service
+````
+
+The following commands will verify that the services are set to start
+at boot time. These should report "enabled".
+
+````sh
+sudo systemctl is-enabled httpd.service
+
+sudo systemctl is-enabled shibd.service
+````
+
+# 11. Restart HTTPD service
+
+```sh
 sudo systemctl restart httpd.service
 ```