Do not fail cell's instance deletion, if it's missing info_cache

Currently the methods in cell messaging are trying to refresh the
instance. However, in some corner cases info_cache is not being
created for instances in ERROR state. This makes the delete
operation, of such instances, to fail, while it should not.

Handling the InstanceInfoCacheNotFound exception and not
re-raising it, for delete operations.

(cherry picked from commit 47898ba8f9526c88a03209dbc35a59d90b79e809)

Conflicts:
        nova/tests/cells/test_cells_messaging.py

Closes-Bug: #1316373
Change-Id: I33c33e3ac1180e8293d950d60fb126e325a2c0cf
Merge "Do not fail cell's instance deletion, if it's missing info_cache" into stable/icehouse