Increase reboot timeout to 15 mins
Increase reboot timeout to 15 mins

This change increases the reboot timeout setting from 240 seconds to
900 seconds as some hardware takes much longer to reboot than others.

Change-Id: I4d17e6c1800f108747c50eb7f29638d28ecb555e
Closes-Bug: #1594882
DocImpact: The default timeout for astute to wait for a node to be
rebooted is now 15 minutes. To manually change this, you can update the
astute.conf file and change the reboot_timeout value to the number of
seconds to wait for a reboot. After updating the configuration file, you
will need to restart the service.

stackforge/fuel-astute,stackforge/fuel-astute