Rewrite RemoteClient.get_boot_time()

This is doing conversion between time formats, while we could really
just stay in seconds. This also fixes the method to work for cirros,
which doesn't know about human readable time formats.

Change-Id: Id123d71adb29d169a26f0dde6af00f292fe61d09
Closes-Bug: #1244823
Merge "Rewrite RemoteClient.get_boot_time()"