Instance var @ssm_skip_resolution only settable as ENV var

[ashleygould]

Currently a user can set ENV['SSM_SKIP_RESOLUTION']. This will disable setting the @client in Uc3Ssm::ConfigResolver instances. But the user can not directly set a ssm_skip_resolution option when calling Uc3Ssm::ConfigResolver.new().

Also, this instance var should be treated strictly as boolean. Just checking if ENV.key?('SSM_SKIP_RESOLUTION') is not sufficient. The user should be able to set ENV['SSM_SKIP_RESOLUTION'] = 'false'.