[change] Making batch imports atomic #551

[stktyagi]

Used transaction.atomic() to ensure all operations inside succeed together. Incase of failure rollback will occure.

Fixes #551

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] 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 #551.

Description of Changes

Added usage of transaction.atomic() functions to introduce atomicity and rollback in case of transaction operation failure.

[coveralls]

coverage: 98.683%. remained the same when pulling 274268e0f2840a891fa06f7ef5c33f7e70048684 on stktyagi:issues/551 into 187e81bf794a2d8d96e344fec5c98823e331c5d2 on openwisp:master.