CI: Make node labels job-specific

Originally, the eligible nodes for a job were labelled only by
"swdev-docker".  So basically any node could run any job.  We had
found that allowing a node to run more than 1 gate at a time was
problematic so we limited the nodes to processing 1 job at a time.
With the creation of the Asterisk 17 branches however, we now have
so many active branches that getting checks and gates through in
a timely manner is problematic when a node can run only 1 job
at a time.

Now the nodes are also labelled by the job type they can run.
For instance: "asterisk-check", "asterisk-gate", etc.  With the
"Throttle Concurrent Builds" plugin, we can now allow a node to
run more than 1 job BUT throttle by job type.  For instance:
  Allow 2 jobs but only 1 asterisk-gate at a time.
Now a node can run 2 checks or 1 check and 1 gate or 1 gate but
not 2 gates at a time.

Change-Id: I2032bf6afbcec5c341d9b852214c0c812d3d6db5

tests/CI/gates.jenkinsfile

@@ -57,7 +57,7 @@ pipeline {
 
 	agent {
 		/* All of the stages need to be performed on a docker host */
-		label "swdev-docker"
+		label "asterisk-gate"
 	}
 
 	stages {

tests/CI/periodics-daily.jenkinsfile

@@ -29,7 +29,7 @@ pipeline {
 
 	agent {
 		/* All of the stages need to be performed on a docker host */
-		label "swdev-docker"
+		label "asterisk-daily"
 	}
 
 	stages {

tests/CI/ref_debug.jenkinsfile

@@ -29,7 +29,7 @@ pipeline {
 
 	agent {
 		/* All of the stages need to be performed on a docker host */
-		label "swdev-docker"
+		label "asterisk-ref-debug"
 	}
 
 	stages {

tests/CI/unittests.jenkinsfile

@@ -59,7 +59,7 @@ pipeline {
 	}
 	agent {
 		/* All of the stages need to be performed on a docker host */
-		label "swdev-docker"
+		label "asterisk-check"
 	}
 
 	stages {