Changed email service parameter to a list of predefined values.

The email service parameter (Gmail, Mailgun, SendGrid, etc.) was free form text when installing from the gallery.

This could leave users confused as to what to enter.

This change makes the email service a drop down list of predefined values.  For now and to the best of my knowledge, I have limited the values to those that have been used successfully with Ghost installations.  We can expand as needed.