installation: duplicate mechanize removal

* Addresses problem with duplicated requirement for mechanize package.
  (closes #1520)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Reported-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
SQLAlchemy: fix for MySQL gone exception handling

* Addresses problem with unclear exception when MySQL is down.
  (closes #1518)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>