Change mongodb extras to lowercase

There's a bug[1] in setuptools where extra names with mixed case
aren't handled correctly. Rather than wait for a fix in setuptools
just change the name to lowercase.

[1] https://bitbucket.org/pypa/setuptools/issues/362/case-sensitive-extras-not-handled-for-dist

DocImpact

Related-Bug: 1479962
Change-Id: I605f1769d85eb18d24783ea25d4c6bedd27cb42d

setup.cfg

@@ -27,7 +27,7 @@ ldap =
   ldappool>=1.0 # MPL
 memcache =
   python-memcached>=1.56
-MongoDB =
+mongodb =
   pymongo>=3.0.2
 bandit =
   bandit>=0.13.2

tox.ini

@@ -11,7 +11,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        .[ldap]
        .[memcache]
-       .[MongoDB]
+       .[mongodb]
 commands = bash tools/pretty_tox.sh '{posargs}'
 whitelist_externals = bash
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
@@ -20,7 +20,7 @@ passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 deps = -r{toxinidir}/test-requirements.txt
        nose
        .[memcache]
-       .[MongoDB]
+       .[mongodb]
 commands =
   nosetests --with-coverage --cover-package=keystone \
       keystone/tests/unit/auth/test_controllers.py \