CB-6889 "Edit json" step in master.cfg fails on Windows

When running medic on windows (either the client and server or just a client), the Edit json step specified in master.cfg fails.
*Reason:* Windows medic install uses the unix utilities from git\bin, the version of sed installed requires specification of backup suffix for -i option without space.
```
  -i[SUFFIX], --in-place[=SUFFIX]
                 edit files in place (makes backup if extension supplied)
```
from sed help.

Fix for [CB-6889](https://issues.apache.org/jira/browse/CB-6889)