install-guide: minor fixes on the glance chapter

Use colons where appropriate.
Fix the formatting of code examples.
Small updates to reflect that some elements must be created/modified.

Change-Id: Icd069533e8f47c9f9f99eb1cfbabc3ad1c3ca427
install-guide: minor fixes

Fix some minor issues in the first chapter of the install guide.

Add a note about installing a backported kernel for Networking on Ubuntu
12.04.

Don't install mysql/mariadb clients on compute and network nodes,
python-mysqldb is enough.

Change-Id: I36e91dce7d2f90b87cd8e725ec80c1e18058753c
Merge "install-guide: minor fixes"
Merge "install-guide: minor fixes on the glance chapter"