Now MySQL is no longer supported, we need to change the docs

Now the rake task takes the path to the default storage directory.

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1. Gems need to be installed before we can run the
  gitlab:workhorse:install task
1. Update gitlab-shell after gitlab-workhorse since it's a manual step
  that needs to be done in `/home/git/gitlab-shell`

Signed-off-by: Rémy Coutable <remy@rymai.me>

This reverts merge request !7909

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[ci skip]

Signed-off-by: Rémy Coutable <remy@rymai.me>

The task will use the canonical repo and the required version.

Signed-off-by: Rémy Coutable <remy@rymai.me>

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

This reverts commit 66d8ef1a, reversing
changes made to 6a8359f3.

The `--with` option was added in Bundler 1.10 but in this case it
shouldn't even be needed: I've tested the original command locally and
the pg gem was installed, so I believe the user that opened the original
MR ran the wrong command (i.e. `sudo -u git -H bundle install --without
development test postgres --deployment` instead of `sudo -u git -H bundle
install --without development test mysql --deployment`.

See discussion at
https://gitlab.com/gitlab-org/gitlab-ce/commit/66d8ef1ad9d5952b7e7f026658f71aafc861950e
for more context.

Add explicit --with statement for postgres and mysql gem groups as necessary. When I followed these instructions, the first command did not install the postgres group and it had to be made explicit.

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