[_sonic_yang_ext.py]: Using python mixin to extend sonic_yang class i… (sonic-net#65)

* [_sonic_yang_ext.py]: Using python mixin to extend sonic_yang class into 2 files.

Changes:
1.) Moved al functions of _sonic_yang_ext.py file in sonic_yang_ext_mixin class.
2.) Use this class in sonic_yang to extend funtionality.

Note: Python mixin are different from parent class since it can access all variables
of the class which is using it.

Signed-off-by: Praveen Chaudhary [email protected]

* [sonic-yang-mgmt]: Resolving LGTM.

Changes:
To address 'import *' related LGTM alerts
1.) Moved all functions of _sonic_yang_ext.py file in sonic_yang_ext_mixin class.
2.) Use this class in sonic_yang to extend funtionality, i.e. python mixin.

Note: Python mixin are different from parent class since it can access all variables
of the class which is using it.

Other LGTM issues
3.) Address Other LGTM minor issues found in below link.
https://lgtm.com/projects/g/Azure/sonic-buildimage/rev/pr-0f82616403c02577e1155347eb5b693a51c76a9e

Signed-off-by: Praveen Chaudhary [email protected]

Author: Praveen Chaudhary