Split functions

Move shared and non-DevStack-specific functions to `functions-common`.  This is
a code move only with some updated comments.  The functions are now
sorted alphabetically within function groups, eg. all git-related functions
are grouped together.  The groups are listed at the top of the file.

'functions' sources 'functions-common' so no additional changes are required
for backward-compatability.

All functions shared with Grenade have also been moved.

functions-common was created from commit e0ed8ea038299952826b27a16753775472f108d8

Change-Id: I73bf7134fd6a60ec1ea44a5bfab08b0569b60ded
Merge "Split functions"