Domain Management: DNS: Minor improvements to SRV Record form

[rads]

• Fix a bug where the form would return a server-side error if the Protocol select box was left in it's default state.
• Change the "Value" field to "Name" to make the purpose more clear.

Testing

1. Go to http://calypso.localhost:3000/domains/manage
2. Click on a domain registration or mapping and select "DNS Editor"
3. Try to add a SRV record but do not change the Protocol option
4. The SRV record should get added successfully.

[rads]

This is actually a duplicate of #1010.