Update global variables of HttpBroadcast so that multiple SparkContexts can coexist

[YanTangZhai]

Update global variables of HttpBroadcast so that multiple SparkContexts can coexist

[SparkQA]

QA tests have started for PR 2058. This patch DID NOT merge cleanly!
View progress: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/18952/consoleFull

[SparkQA]

QA results for PR 2058:
- This patch FAILED unit tests.

For more information see test ouptut:
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/18952/consoleFull

[SparkQA]

QA tests have started for PR 2058 at commit a1b4efb.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2058 at commit a1b4efb.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class HttpBroadcastContainer extends Serializable with Logging

[SparkQA]

QA tests have started for PR 2058 at commit 2646398.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2058 at commit 2646398.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class HttpBroadcastContainer extends Serializable with Logging

[YanTangZhai]

#2059