(maint) Simplify the CA use of the autosign setting

Commit bca628dc adds proper conversion of autosign input values to
booleans, but Puppet::SSL::CertificateAuthority was still trying to
handle string values. This commit simplifies the checking logic and lets
the autosign setting handle data normalization.
Merge pull request #2129 from adrienthebo/master-maint-simplify_autosign_settings_use

(maint) Simplify the CA use of the autosign setting