GitLab

 - Verify the creation of ByteString variable using
   UA_Server_DataSetReader_addTargetVariables

Change-Id: Ia9acb07b5eca73708df9bc17b7f6bb5aa81e8790
Signed-off-by: Thangavaila Kalyani Thangaraja <thangavaila.k@kalycito.com>

tutorial_pubsub_subscribe.c

 - Copy data of specified length using UA_snprintf statement
   in ua_pubsub.c
 - Change UA_LOG_WARNING to UA_LOG_ERROR when variable node
   addition fails in UA_Server_DataSetReader_addTargetVariables

Change-Id: I6c5947f7330d24a31d5d00d186015d6fc9f5f720
Signed-off-by: Thangavaila Kalyani Thangaraja <thangavaila.k@kalycito.com>

fixes #3048

The container can be built with:
`docker build -t open62541 . && docker run -p 4840:4840 open62541`

fixes #3001

Add badge for azure pipeline

1. generateChildNodeId: Used to define NodeIds for automatically generated nodes during recursive node instantiation
2. createOptionalChild: Used to define whether an optional child should be created during recursive node instantiation

Function removeSession should call access control closeSession as the only one.

ci: Fix azure pipeline

 - Remove the redundant listing of DiscoveryUrls in
   FindServer service
Signed-off-by: Jayanth Velusamy <jayanth.v@kalycito.com>

This moves most of the builds from Appveyor to the more efficient
and significantly faster Azure Pipelines system.

For now Azure Pipelines by default only supports VS2015 and VS2017,
therefore the builds for VS2013 and VS2008 still remain on Appveyor.

These can be transferred at a later stage.

Sometimes travis failed with
```
/home/travis/build/open62541/open62541/build/src_generated/open62541/nodeids.h:6:0: error: unterminated #ifndef

 #ifndef UA_NODEIDS_NS0_H_

 ^
```
if the plugins object was created before namespace generation finished.