Support for relocatable RPMs

[fsat]

Save the value of RPM_INSTALL_PREFIX into application's sysconfig file.

Upon application start up, look for this value, else use the chdir value supplied via sbt templates.

I have two options in detecting where the RPM package is installed.

Option 1: Use RPM_INSTALL_PREFIX provided during installation process

The RPM_INSTALL_PREFIX environment variable is only provided during the install time. This value would need to be persisted so it can be retrieved during application start-up.

Option 2: Use rpm command

The command rpm -q --queryformat "%{INSTPREFIXES}" ${{app_name}} will print out the location where the package is installed. This command works for both Redhat 6 and 7.

I have decided to go with Option 1 because we are using the values provided to us by the operating system instead of trying to deduce this value ourselves.

The downside of this approach is:

• I am persisting the value of RPM_INSTALL_PREFIX in the /etc/sysconfig/${{app_name}}.
• I need to replace this information at re-install time or upgrade time.

@muuki88 and @edwardcallahan - can I get your thoughts on my approach please? Happy to change if you think Option 2 is a better way to move forward.

[fsat]

Fixes #659 and https://github.com/typesafehub/conductr/issues/790

[fsat]

I have done a manual test with the following steps:

• Override the preinst and start with the original content from sbt-native-packager + changes I have introduced applied.
• Generate rpm package
• Install rpm package into default path, e.g. sudo rpm -i conductr-0.1.0-1.noarch.rpm
• Ensure process are started correctly with package files created into the correct locations.
• Uninstall the package.
• Re-install rpm package into relocated path, e.g. sudo rpm -i conductr-0.1.0-1.noarch.rpm --relocate /usr/share=/opt
• Ensure process are started correctly with package files created into the correct locations.

The manual test has been done on Redhat 7.

[huntc]

Using -z for tests is better.

Otherwise LGTM - @muuki88 - WDYT?

[fsat]

Used -n instead since I'd like to check for non-empty string, also shorten the if clause where possible.

[fsat]

Manual test also done successfully on Redhat 6.

[muuki88]

LGTM.