Refactor steps/connection.py

To be merged after !1270 (merged)

Part of #62

There are many instances of basically the same step, should be unified in feature files:

* Bring up connection "con"    # should be replaced by
* Bring "up" connection "con"


* Bring down connection "con"  # should be replaced by
* Bring "down" connection "con"

* Bring down connection "con" ignoring error # down should be param
* Bring up connection "con" ignoring error   # up should be param

* Bring up connection "con" for "eth1" device  # can be the same function as * Bring "up" connection
* Start generic connection "con" for "eth1"    # sounds weird, use Bring version

All * Add, * Modify and * Bring steps use nmci.process.nmcli() now. This change uncovered some bugs in feature files (mostly missing cleanups).

@RunTests:*