MEDIUM: Send email alerts when servers are marked as UP or enter the drain state

MEDIUM: Send email alerts when servers are marked as UP or enter the drain state

This is similar to the way email alerts are sent when servers are marked as
DOWN.

Like the log messages corresponding to these state changes the messages
have log level notice. Thus they are suppressed by the default email-alert
level of 'alert'. To allow these messages the email-alert level should
be set to 'notice', 'info' or 'debug'. e.g:

email-alert level notice

"email-alert mailers" and "email-alert to" settings are also required in
order for any email alerts to be sent.

A follow-up patch will document the above.

Signed-off-by: Simon Horman <horms@verge.net.au>