Fix gate error "mysqladmin: command not found"

mysqladmin package is required in tools/test-setup.sh
so adding bindep.txt file for this binary dependencies

- http://lists.openstack.org/pipermail/openstack-discuss/2019-June/007272.html

Change-Id: Iff79081403fbc26442165ff5feb300bc317b0dfc

bindep.txt

@@ -0,0 +1,5 @@
+# This is a cross-platform list tracking distribution packages needed by tests;
+# see http://docs.openstack.org/infra/bindep/ for additional information.
+
+mysql-client [platform:dpkg]
+mysql-server [platform:dpkg]

tox.ini

@@ -63,3 +63,14 @@ commands =
 [testenv:releasenotes]
 basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+
+[testenv:bindep]
+# Do not install any requirements. We want this to be fast and work even if
+# system dependencies are missing, since it's used to tell you what system
+# dependencies are missing! This also means that bindep must be installed
+# separately, outside of the requirements files, and develop mode disabled
+# explicitly to avoid unnecessarily installing the checked-out repo too (this
+# further relies on "tox.skipsdist = True" above).
+usedevelop = False
+deps = bindep
+commands = bindep test