Add option to configure editor to use

[shahzebsiddiqui]

Currently one can't configure the editor to use when using commands like buildtest config edit to edit configuration file or buildtest buildspec edit <test> or buildtest buildspec edit-file <buildspec>.

By default it will look at value of environment EDITOR if its set it will use it otherwise it will resort to vi. This can be problematic for users that may not have vi or perhaps they don't like their editor.

One option is to allow one to specify editor to use using one of the following method

1. Environment Variable perhaps BUILDTEST_EDITOR?
2. Configuration property perhaps editor in buildtest configuration which specify full path to editor for instance editor: /usr/bin/vim
3. Another option is to specify editor on command line buildtest --editor /usr/bin/vim config edit

In this case the order of preference is following

1. Command Line
2. Environment Variable
3. Configuration File

Let me know your thoughts @Xiangs18 @JeffreyFG and whether this would be good approach.

This will impact the following files

• https://github.com/buildtesters/buildtest/blob/devel/buildtest/cli/config.py
• https://github.com/buildtesters/buildtest/blob/devel/buildtest/cli/buildspec.py