Correct the tox option for skipping sdist generation

The tox option to skip source distribution building is skipsdist,
but this seems to be often misspelled skipdist instead, which gets
silently ignored and so does not take effect. Correct it
everywhere, in hopes that new projects will finally stop copying
this mistake around.

See https://tox.readthedocs.io/en/latest/config.html#conf-skipsdist
and tox-dev/tox#1388 for details.

Change-Id: Ieee8041dd177b28d74a0f175fb6df340d158c3c9

tox.ini

@@ -236,7 +236,7 @@ setenv =
 # package available only for /usr/bin/python interpreter
 basepython = {env:IR_PYTHON:python3}
 usedevelop = false
-skipdist = true
+skipsdist = true
 skip_install = true
 sitepackages = true
 whitelist_externals =
@@ -294,7 +294,7 @@ commands =
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
 usedevelop = false
-skipdist = true
+skipsdist = true
 skip_install = true
 
 
@@ -313,7 +313,7 @@ commands =
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
 usedevelop = false
-skipdist = true
+skipsdist = true
 skip_install = false
 
 
@@ -328,7 +328,7 @@ commands =
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
 usedevelop = false
-skipdist = true
+skipsdist = true
 skip_install = true
 
 
@@ -342,7 +342,7 @@ basepython = {[testenv:py3]basepython}
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
 usedevelop = false
-skipdist = true
+skipsdist = true
 skip_install = true
 deps = bindep
 commands = bindep {posargs:test}