(maint) Update VMPooler files with fixes for Rubocop violations

This commit updates vmpooler.rb, api.rb and providers.rb with style changes as
per rubocop style violations.

This commit also updates the rubocop configuration to always use LF line endings
even on Windows as rubocop was expecting CRLF even though git is configured
for LF.

.rubocop.yml

@@ -9,6 +9,8 @@ AllCops:
     - 'scripts/**/*'
     - 'spec/**/*'
     - 'vendor/**/*'
+    - Gemfile
+    - Rakefile
 
 Style/Documentation:
   Enabled: false
@@ -58,3 +60,7 @@ Style/ConditionalAssignment:
 Next:
   Enabled: false
 
+# Enforce LF line endings, even when on Windows
+Style/EndOfLine:
+  EnforcedStyle: lf
+