Adds missing dependency to the documentation
Adds missing dependency to the documentation

Trove depends on MySQL-python package, to compile and install it is
required libmysqlclient-dev, otherwise the installation procedure
described in 'doc/source/dev/manual_install.rst' will fail in recently
installed boxes.

Closes-Bug: #1261525
Change-Id: I0cc325eb7efb056efe9682fa3478795ae025bfa1

fabian4/trove,zhangg/trove,mmasaki/trove,zhujzhuo/openstack-trove,zhangg/trove,fabian4/trove,zhujzhuo/openstack-trove,mmasaki/trove,denismakogon/trove-guestagent,denismakogon/trove-guestagent,openstack/trove,mmasaki/trove,hplustree/trove,cp16net/trove,changsimon/trove,hplustree/trove,redhat-openstack/trove,cp16net/trove,fabian4/trove,changsimon/trove,changsimon/trove,redhat-openstack/trove,zhujzhuo/openstack-trove,openstack/trove,redhat-openstack/trove,cp16net/trove