Move flake8 config into flake8 section in tox.ini

Sometimes, it would be convenient to run flake8 directly rather than
`tox -e flake8` and the flake8 section in tox.ini will apply to both of
them.

In addition, exclude ./.env as well.

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

tox.ini

@@ -7,7 +7,14 @@
 envlist = flake8, py27, py3
 
 [flake8]
+ignore = E731
 max-line-length = 100
+exclude =
+    ./.tox
+    ./.git
+    ./module_build_service/migrations
+    ./build
+    ./.env
 
 [testenv]
 usedevelop = true
@@ -30,7 +37,7 @@ commands =
 basepython = python2
 skip_install = true
 deps = flake8
-commands = flake8 --ignore E731 --exclude ./.tox,./.git,./module_build_service/migrations,./build
+commands = flake8
 
 [testenv:bandit]
 basepython = python2