Use wait condition when waiting for undercloud

We can now use the Heat stack wait condition to determine that the
undercloud node is up and fully initialized.

Change-Id: Ie9f026288e783188dca80421f7282283bea3511c
Preserve USE_CACHE value in devtest_setup.sh

Preserve the value of USE_CACHE environment variable set in
devtest.sh, but let it be overridable by a command line argument
passed to devtest_setup.sh

Change-Id: Ia98077d4ec3d20fa81b6fe9ab24cf066c1f6a7a4
Cleanup docs

 - add missing section titles
 - fix indentation in code blocks
 - move squid footnote to devtest_setup.sh
 - fix all remaining warnings

Change-Id: I61592ab52907fce64c0f39b88500c43b2ce55689