Proposed fix for JENKINS-24401

The Depot class now has a method for obtaining the server's timezone GMT
offset. This offset is then applied to the changelist's submit time. Thanks to
that, the submit time of a changelist in the build's changelog appears in the
Jenkins server's timezone.
Address comments made in review

There are now no changes to the public API. When ChangelistBuilder does not
contain a valid reference to a Depot object, fall back to the default behaviour
regarding changelist timestamp parsing.
Added a simple support of global Perforce credentials

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Fixes

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Use default credentials in validation forms (if the job-specific user is missing)

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #56 from xavery/server-timezone

[FIXED JENKINS-24401] - Take server timezone offset into account when displaying change date
Merge pull request #57 from synopsys-arc-oss/custom-global-credentials

Added a simple support of global Perforce credentials
Merge branch 'master' of https://github.com/jenkinsci/perforce-plugin
Merge branch 'master' into release_prep
Refactor https://github.com/jenkinsci/perforce-plugin/pull/56

Changes:
* return null when Depot::getServerTimezone() fails to parse the timezone
* handle nulls in ChangeListBuilder::build()
* Propagate parsing errors to the upper level ChangeListBuilder::parseDateWithTimezone()
* code formatting, etc.

CC: @xavery

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>