Imported Translations from Transifex

Change-Id: I6f29f21c031370a64c8ae75652936762170406f9
Allow gateway address to be unset for an existing subnet

Validation of the gateway IP address was taken place even
if the gateway IP was set to None; this was causing an
AddrFormatError exception to be raised by method
'_validate_gw_out_of_pools'. With this change we skip
the validation in case the gateway is set to None.

Fixes bug #1178273

Change-Id: Ib84378a4fd2cefdbbcacce695abbfaf82c647dd3
Add cover/ to .gitignore

When we run "tox -e cover" to generate coverage report, testr will make
"cover" directory which contains the same html in "covhtml" made by
"./run_test.sh --coverage".  So, make git ignore this directory.

Change-Id: I829667a1ceb55418a5695545ad543a53f63821ba
Merge "Imported Translations from Transifex"
Merge "Add cover/ to .gitignore"