Add backup member tests

This patch introduces backup member tests to the octavia-tempest-plugin.

There is one new scenario test:
 - test_backup_member

And 5 new API tests:
 - test_ipv4_HTTP_LC_backup_member_create
 - test_HTTP_LC_backup_member_show
 - test_HTTP_LC_backup_member_update
 - test_HTTP_LC_backup_member_delete
 - test_HTTP_LC_backup_member_list

Change-Id: If7c255ed24cf310a6a0db1a70fee104461e79d61
diff --git a/octavia_tempest_plugin/common/constants.py b/octavia_tempest_plugin/common/constants.py
index 8ef8d94..725f20f 100644
--- a/octavia_tempest_plugin/common/constants.py
+++ b/octavia_tempest_plugin/common/constants.py
@@ -112,6 +112,7 @@
 
 # Other constants
 ACTIVE = 'ACTIVE'
+PAUSED = 'PAUSED'
 PENDING_UPDATE = 'PENDING_UPDATE'
 ADMIN_STATE_UP_TRUE = 'true'
 ASC = 'asc'
@@ -126,6 +127,7 @@
 SINGLE = 'SINGLE'
 ACTIVE_STANDBY = 'ACTIVE_STANDBY'
 SUPPORTED_LB_TOPOLOGIES = (SINGLE, ACTIVE_STANDBY)
+BACKUP_TRUE = 'true'
 
 # Protocols
 HTTP = 'HTTP'