Consistently trim string parameters

None of the string parameters can meaningfully have leading or
trailing whitespace, so trim them all for consistency.

Change-Id: I6a4cb55daa60c3e7a03f4e4e0f60678bf06a963b
diff --git a/mbedtls-dockerfile-builder.yaml b/mbedtls-dockerfile-builder.yaml
index 07d73ed..3c0db9a 100644
--- a/mbedtls-dockerfile-builder.yaml
+++ b/mbedtls-dockerfile-builder.yaml
@@ -76,12 +76,12 @@
         default: ssh://git@github.com/Mbed-TLS/mbedtls-test.git
         description: Mbed TLS test repo/fork
         name: MBED_TLS_TEST_REPO
-        trim: 'false'
+        trim: true
     - string:
         default: main
         description: Branch or commit to build Dockerfile from
         name: MBED_TLS_TEST_BRANCH
-        trim: 'false'
+        trim: true
     - bool:
         default: false
         description: Build the Ubuntu 16.04 Docker image
@@ -90,7 +90,7 @@
         default: ubuntu-16.04-testing
         description: Tag for the Ubuntu 16.04 Docker image being built
         name: TAG_FOR_16_04_DOCKERFILE
-        trim: 'false'
+        trim: true
     - bool:
         default: false
         description: Build the Ubuntu 18.04 Docker image
@@ -99,7 +99,7 @@
         default: ubuntu-18.04-testing
         description: Tag for the Ubuntu 18.04 Docker image being built
         name: TAG_FOR_18_04_DOCKERFILE
-        trim: 'false'
+        trim: true
     - bool:
         default: false
         description: Build the Ubuntu 20.04 Docker image
@@ -108,7 +108,7 @@
         default: ubuntu-20.04-testing
         description: Tag for the Ubuntu 20.04 Docker image being built
         name: TAG_FOR_20_04_DOCKERFILE
-        trim: 'false'
+        trim: true
     project-type: pipeline
     properties:
     - speed-durability: