pluginfeature: Allow updating initial rank of plugin feature

Introducing "GST_PLUGIN_FEATURE_RANK" environment variable in order for users
to adjust rank of plugin(s) via environment.

A "feature" and "rank" key-value pair should be separable by ":",
and each key-value pair is recognized per "," delimiters. The rank
can be a numerical value or one of pre-defined rank values
such as "NONE", "MARGINAL", "SECONDARY", and "PRIMARY"

Example)
- GST_PLUGIN_FEATURE_RANK=qtdemux:256,h264parse:NONE
Set rank of qtdemux plugin to 256 (primary) and 0 (none) for h264parse.
22 jobs for feature-rank-env in 69 minutes and 32 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #507545
gst indent

00:00:56

passed #507544
manifest

00:00:46

 
  Build
passed #507551
build cerbero android universal

00:16:01

passed #507552
build cerbero cross win32

00:11:11

passed #507553
build cerbero cross win64

00:05:24

passed #507550
build cerbero fedora x86_64

00:08:55

manual #507555
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #507554
gst-macos-10.14
build cerbero macos x86_64

00:13:28

passed #507546
build fedora x86_64

00:06:38

passed #507547
build nodebug fedora x86_64

00:05:29

passed #507548
build static fedora x86_64

00:12:52

passed #507549
build static nodebug fedora x86_64

00:12:20

 
  Test
passed #507556
check fedora

00:04:27

passed #507557
integration testsuites fedora

00:21:49

passed #507562
valgrind bad

00:27:29

passed #507559
valgrind base

00:12:24

passed #507558
valgrind core

00:14:39

passed #507563
valgrind ges

00:08:43

passed #507560
valgrind good

00:38:45

passed #507561
valgrind ugly

00:05:09

 
  Integrate
passed #507564
android universal examples

00:09:37

passed #507565
documentation

00:05:44