Catch exceptions from nova client in poll_and_publish

Ceilometer compute agent dies if nova client raises an exception while it is
retrieving server instances. This might happen e.g. when some OpenStack API is
temporarily unavailable

Conflicts:
	tests/agentbase.py
	tests/compute/test_manager.py

Fixes LP Bug #1218889
Change-Id: I808dcfae18d23240f8e095d6c97c8dede7dede8f
(cherry picked from commit 0e01da763ab7b3783ae86aa6582c38ebdc6e2378)