Addressing most of Alvaro's concerns per code review
* Reformatted to 120 chars max per line.
* Small tweak made to template.
* Fixed resolved-cache-mode default value: intended to be "" (system
default), but was left as "yes" as it was set before I added the ""
option.
* Changes to resolved.conf don't require a reboot; they just require a
service restart. Thus, am using _render_resource() (new helper
function) instead of _render_boot_resource() for rendering the file,
and have added a service restart as a side effect of the config
changes, the same as is done for cpufrequtils.
The restart ideally should only be done if there were actually any
changes to the config file; I will add this in the next commit.
Note: the raid-autodetection log message logged that "off" was a valid
value, but that is incorrect based upon the set of values actually
checked against.
* Copied the systemd.conf-related code and reworked to fit controlling
resolved.conf.
* Updated the test from the previous commit and added extra tests to
cover the new code.
* Removed "cosmic" from the list of test series'; while expected to
fail, this failing test added considerable time to running the whole
functional test suite due to the 10 minute TIMEOUT constant.
* Refactored 2 tests related to testing invalid config values into one
parameterized test, then added values to test the resolved-cache-mode
config value.
