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
Merge "Adds missing dependency to the documentation"