CLI fix offline command save stacks to tars

[riuvshin]

What does this PR do?

fixes two issues related to CLI offline mode during stack images saving

first issue:
if image name has more than 1 / symbol like registry.centos.org/che-stacks/vertx save to tar will fail:

...
INFO: (che offline): Saving /Users/roman/development/codenvy_projects/.che/backup/eclipse_ubuntu_jre.tar...
INFO: (che offline): Saving /Users/roman/development/codenvy_projects/.che/backup/eclipse_ubuntu_python.tar...
INFO: (che offline): Saving /Users/roman/development/codenvy_projects/.che/backup/eclipse_ubuntu_wildfly8.tar...
INFO: (che offline): Saving /Users/roman/development/codenvy_projects/.che/backup/registry.centos.org_che-stacks/vertx.tar...
/scripts/base/commands/cmd_offline.sh: line 142: /data/backup/registry.centos.org_che-stacks/vertx.tar: No such file or directory
ERROR: Docker was interrupted while saving /data/backup/registry.centos.org_che-stacks/vertx.tar

to fix this I;ve updated sed pattern to replace all / symbols with _

second issue:
if images-stacks file has any empty line like this https://github.com/eclipse/che/blob/master/dockerfiles/cli/version/5.11.2/images-stacks#L24
that will cause an error during iterating over array which is made from that file:

...
INFO: (che offline):   Image eclipse/ubuntu_wildfly8 already saved...skipping
INFO: (che offline):   Image registry.centos.org/che-stacks/vertx already saved...skipping
/scripts/base/commands/cmd_offline.sh: line 133: 1: unbound variable

to fix this I;ve added check on empty strings:

What issues does this PR fix or reference?

#5268

Changelog

CLI fix offline command save stacks to tars

[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/2737/