Allow multi-line config items in meta-section of local.conf

It would behave such as the contents from each meta-section in
local.conf is copied to the destination files. One exception is the multiline
options not grouped together. In that case, the contents will be grouped
together in its destination config file.

Check tests/test_config.sh for examples.

Change-Id: I8c046b558eeb98ed221f6f1a59182d4179956ced
Partial-Bug: #1374118
Merge "Allow multi-line config items in meta-section of local.conf"
Merge commit 'refs/changes/03/127603/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch