(PDB-954) Support a fallback database

This commit moves from a combo 'server' and 'port' config to a
'server_urls' config that contains the full path to one or more PuppetDB
URLs. The old server/port format is still supported (internally it is
converted to a URL to conform with the new syntax).

This commit also supports a new 'server_url_timeout' config. If the
PuppetDB instance has responded in the specified number of
seconds (defaults to 30) it will fail and roll to the next PuppetDB
url (if one is provided).
Merge pull request #1129 from wkalt/ticket/master/pdb-954-fallback-acceptance

Ticket/master/pdb 954 fallback acceptance