Environment variables expansion fails if variable name contains dot/dash

[snallami]

For version 2.5, environment variable names can contain dot or dashes. Project keys like $sonar.projectkey in Jenkins configuration are quite common.

New changes broke existing configuration if job is configured with project keys like $sonar.projectkey. Getting below error during build
"java.net.URISyntaxException: Illegal character in query at index 62: https://foo.com/api/events?resource=${sonar.projectKey}&format=json&categories=Alert "

Per Open Group , pretty much any character is allowed for environment variable name except equals. However we need to consider compatibility with all utilities as well.

This PR address issue with dot and/or dash characters in environment variable names.

[snallami]

BTW modified regex have below rules.

1. first characters can be from set [a-zA-Z_]
2. number (0-9), dot(.) and dash(-) cannot be first character

[alexanderrtaylor]

This works locally for me and solves JENKINS-47321 which was marked as solved