luci-app-acme: validation_method options: use the same string as tab names

luci-app-acme: validation_method options: use the same string as tab names

For option names use a longer description that is already used for tabs that shows the method-specific options:
'Webroot' to the 'Webroot Challenge Validation'
'DNS' to 'DNS Challenge Validation'

This allows avoiding duplicated strings.

Signed-off-by: Sergey Ponomarev <[email protected]>