BootTrait - Change default boot mode. Prefer CmsBootstrap.

Most cv subcommands have the default bootstrap flag `--level=full|cms-full`,
which automatically decides whether to use the older `full` (`Bootstrap.php`) or
newer `cms-full` (`CmsBootstrap.php`).  This commit changes the preference.

Before:

* Default to the older `full`
* The env-var `CIVIRM_BOOT` implies a preference for `cms-full`.

After:

* Default to the newer `cms-full`
* The env-vars `CIVIRM_BOOT`, `CIVICRM_SETTINGS`, and `CIVICRM_UF` imply a
  preference for `cms-full`, `full`, and `full` (respectively).

Comment:

All three of these variables already exist.  What changes here is that they
influence whether `full|cms-full` is interpreted as `full` or `cms-full`.

Author: totten

src/Util/BootTrait.php

@@ -33,7 +33,18 @@ public function boot(InputInterface $input, OutputInterface $output) {
     }
 
     if ($input->getOption('level') === 'full|cms-full') {
-      $input->setOption('level', getenv('CIVICRM_BOOT') ? 'cms-full' : 'full');
+      if (getenv('CIVICRM_UF') === 'UnitTests') {
+        $input->setOption('level', 'full');
+      }
+      elseif (getenv('CIVICRM_BOOT')) {
+        $input->setOption('level', 'cms-full');
+      }
+      elseif (getenv('CIVICRM_SETTINGS')) {
+        $input->setOption('level', 'full');
+      }
+      else {
+        $input->setOption('level', 'cms-full');
+      }
     }
 
     if (getenv('CIVICRM_UF') === 'UnitTests' && preg_match('/^cms-/', $input->getOption('level'))) {