Fix invalid escape sequence warnings

Starting with py36, DeprecationWarnings are raised for special
characters in strings that are not being used as valid escape sequences.
This addresses a couple instances we had for string replacement by
changing the replacement strings to be raw strings.

Change-Id: I93291e978aacaf3f639f32174605528f0038f13a
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

os_brick/initiator/connectors/iscsi.py

@@ -348,7 +348,7 @@ class ISCSIConnector(base.BaseLinuxConnector, base_iscsi.BaseISCSIConnector):
         ip, port = connection_properties['target_portal'].rsplit(':', 1)
         # NOTE(geguileo): I don't know if IPv6 will be reported with []
         # or not, so we'll make them optional.
-        ip = ip.replace('[', '\[?').replace(']', '\]?')
+        ip = ip.replace('[', r'\[?').replace(']', r'\]?')
         out = self._run_iscsiadm_bare(['-m', 'discoverydb',
                                        '-o', 'show',
                                        '-P', 1])[0] or ""