Catch exceptions from nova flavor-list calls

* nodepool/provider_manager.py(_getFlavors): If a provider nodepoold
is configured to use goes offline or otherwise becomes unreachable,
novaclient will bubble up ConnectionError exceptions from the
requests library. Catch these to prevent them from shorting out the
main loop, which would otherwise keep new nodes from building on any
working providers as well.

Change-Id: I924d2c21e6e30f760f641223384726d8f9ed122a
Closes-Bug: #1269001
Merge "Catch exceptions from nova flavor-list calls"