Plugins: update example plugins use of the interface module

Commit 88cd8261 moved the plugin interfaces to `avocado/core` under the
name `plugin_interfaces.py`.  The example plugins were not updated, so
let's fix it now.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

examples/plugins/job-pre-post/mail/avocado_job_mail.py

@@ -3,7 +3,7 @@ import smtplib
 from email.mime.text import MIMEText
 
 from avocado.core.settings import settings
-from avocado.plugins.base import JobPre, JobPost
+from avocado.core.plugin_interfaces import JobPre, JobPost
 
 
 class Mail(JobPre, JobPost):

examples/plugins/job-pre-post/sleep/avocado_job_sleep.py

@@ -2,7 +2,7 @@ import time
 import logging
 
 from avocado.core.settings import settings
-from avocado.plugins.base import JobPre, JobPost
+from avocado.core.plugin_interfaces import JobPre, JobPost
 
 
 class Sleep(JobPre, JobPost):