[Fix] Merging with wrong format now raises Validation error #351

[stktyagi]

An attempt to merge config with wrong format will now raise a validation error instead of failing.

Fixes #351

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [x] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #351

Description of Changes

Added conditionals to check if two template types can be merged and if not raise validation error instead of failing.

[coveralls]

coverage: 99.178%. remained the same when pulling a9969bfdc4edd386715023db2fa7d726b829263b on stktyagi:master into 43647b69579ad02e5ffa050b3898c14ea4f31292 on openwisp:master.