Change the percentage of var and temp to spare more for system

Cinder volume is going to be created on system partition if
no other block device is found. Tempest needs at least 5g for
volume tests, so we spare more for system partition. Also fixed
some typo in message template.

Change-Id: I490ff31ae5f6ba6ed1abce64a5237da92294b360
Merge "Change the percentage of var and temp to spare more for system"