mssql: allow connect to to datasource\namedInstance

There used to be explicit check which forbids connection via dataSource = `host\<instance name>`. That seems to be working fine now (tested with SQL Sever Express 2014) so I'm removing this restriction.

Example connection definitions which work fine with named instances:


* Via backslash prefix:
```
!|Connect|myhost\myinstance|myuser|mypassword|mydbname|
```

* By setting parameter `instanceName=...`:
```
!|Connect|myhost;instanceName=myinstance|myuser|mypassword|mydbname|
```
Connection to named instance requires MS SQL Server Browser service to be up and running and allowed by firewalls (if any). Each database should be also configured to accept TCP/IP connections (including allowance in firewall if any).