Funnel all calls to LogFiles>utils.get_log_entries

This affects issue #503 indirectly because it is the first
step in allowing us to disable log lookup globally or limit
max size of logs read.

Also this might allow us in the future to switch to alternative
backends.
status/rest: Use utils.get_log_entries internally
Make log access configurable

This patch creates a new setting 'enable_local_logs' which
if set to false will disable all reading of log files.

This will help in cases like #495 where log access is a supposed
CPU hog for the adagios interface, and makes it simple to turn log
access off when troubleshooting performance issues.

Also affects issue #503 as one part of solving top_alert_producer issues.
Merge pull request #507 from opinkerfi/feature-logfiles-settings

Funnel all calls to LogFiles>utils.get_log_entries
Merge branch 'master' into feature-livestatus-limit

Conflicts:
	adagios/status/utils.py