Add shelve/unshelve test of nova APIs

The shelve feature powers off and gets a vm snapshot. Also the unshelve
feature restarts a vm instance from the snapshot.
This patch adds an API test for shelve/unshelve features.

Change-Id: Ica8e8c864371c0e2018b2c7df9c6ecbbeddaf5df
Closes-Bug: 1235695
Merge "Add shelve/unshelve test of nova APIs"