Change type of ssh_port option from Str to Int

The type of CONF option virtual_power_ssh_port was incorrectly defaulted
to Str. This can cause Paramiko to raise when casting to %d.

Fixes bug 1157824.

Change-Id: I30ddd1ff0da45f8392085249f1bd2a539b201a7e
Merge "Change type of ssh_port option from Str to Int"