fix(convert): handle conversion of multiple ENV in oneline in Dockerfile

[darshilgit]

What type of PR is this?
Bug

Which issue does this PR fix:

# kind1: ENV needs a default value
ENV foo /bar
# kind2: you can also write
ENV foo=/bar
# kind3: multiple ENV in oneline
ENV TEST_PATH="/usr/share/test/bin:$PATH" \
    TEST_PATHS_CONFIG="/etc/test/test.ini" \
    TEST_PATHS_DATA="/var/lib/test" \
    TEST_PATHS_HOME="/usr/share/test" \
    TEST_PATHS_LOGS="/var/log/test" \
    TEST_PATHS_PLUGINS="/var/lib/test/plugins" \
    TEST_PATHS_PROVISIONING="/etc/test/provisioning"

The PR adds support for the 3rd kind

What does this PR do / Why do we need it:
For the stacker convert to handle Dockerfile with ENV defined in the form:

ENV TEST_PATH="/usr/share/test/bin:$PATH" \
    TEST_PATHS_CONFIG="/etc/test/test.ini" \
    TEST_PATHS_DATA="/var/lib/test" \
    TEST_PATHS_HOME="/usr/share/test" \
    TEST_PATHS_LOGS="/var/log/test" \
    TEST_PATHS_PLUGINS="/var/lib/test/plugins" \
    TEST_PATHS_PROVISIONING="/etc/test/provisioning"

If an issue # is not available please add repro steps and logs showing the issue:

Testing done on this change:

 ✓ convert a Dockerfile

1 test, 0 failures

Automation added to e2e: enhanced test/convert.bats

Will this break upgrades or downgrades?
No

Does this PR introduce any user-facing change?: No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rchincha]

lgtm