Enhance ceilometer statistics command with --period

Currently, if don't specify query parameter in ceilometer statistics
command, ceilometer statistics with --period will not work. This change
is trying to fix this issue.

Change-Id: I8723bad11d5c452c2834e33df9bb01ebdc6ce9ce
Fixs: bug #1202658
Merge "Enhance ceilometer statistics command with --period"