[show][config] cli refactor for muxcable with abstract class implemen…

…tation from vendors (#1722) (#1782)

Merging this in 202012 branch because there was a merge conflict in original PR while cherry-picking
#1722

#### What I did
this PR adds support for cli refactor which is done for abstract class implementation of muxcable which is provided by vendors.
detailed design doc can be found sonic-net/SONiC#757

#### How I did it
Added the changes in muxcable.py in show and config . 

#### How to verify it
add unit-tests as well as run on a Arista-7050cx3 switch, run unit tests. 

no change in outputs, just the logic has changed.
Signed-off-by: vaibhav-dahiya <[email protected]>